Create safe sender lists in EOP

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Applies to

If you're a Microsoft 365 customer with mailboxes in Exchange Online or a standalone Exchange Online Protection (EOP) customer without Exchange Online mailboxes, EOP offers multiple ways of ensuring that users will receive email from trusted senders. Collectively, you can think of these options as safe sender lists.

The available safe sender lists are described in the following list in order from most recommended to least recommended:

  1. Allow entries for domains and email addresses (including spoofed senders) in the Tenant Allow/Block List.
  2. Mail flow rules (also known as transport rules).
  3. Outlook Safe Senders (the Safe Senders list that's stored in each mailbox that affects only that mailbox).
  4. IP Allow List (connection filtering)
  5. Allowed sender lists or allowed domain lists (anti-spam policies)

The rest of this article contains specifics about each method.

Important

Messages that are identified as malware or high confidence phishing are always quarantined, regardless of the safe sender list option that you use. For more information, see Secure by default in Office 365.

Be careful to closely monitor any exceptions that you make to spam filtering using safe sender lists.

Always submit messages in your safe sender lists to Microsoft for analysis. For instructions, see Report good email to Microsoft. If the messages or message sources are determined to be benign, Microsoft can automatically allow the messages, and you won't need to manually maintain the entry in safe sender lists.

Instead of allowing email, you also have several options to block email from specific sources using blocked sender lists. For more information, see Create block sender lists in EOP.

Use allow entries in the Tenant Allow/Block List

Our number one recommended option for allowing mail from senders or domains is the Tenant Allow/Block List. For instructions, see Allow or block email using the Tenant Allow/Block List.

Only if you can't use the Tenant Allow/Block List for some reason should you consider using a different method to allow senders.

Use mail flow rules

Note

You can't use message headers and mail flow rules to designate an internal sender as a safe sender. The procedures in this section work for external senders only.

Mail flow rules in Exchange Online and standalone EOP use conditions and exceptions to identify messages, and actions to specify what should be done to those messages. For more information, see Mail flow rules (transport rules) in Exchange Online.

The following example assumes you need email from contoso.com to skip spam filtering. To do this, configure the following settings:

  1. Condition: The sender > domain is > contoso.com.

  2. Configure either of the following settings:

    • Mail flow rule condition: A message header > includes any of these words > Header name: Authentication-Results > Header value: dmarc=pass or dmarc=bestguesspass.

      This condition checks the email authentication status of the sending email domain to ensure that the sending domain is not being spoofed. For more information about email authentication, see SPF, DKIM, and DMARC.

    • IP Allow List: Specify the source IP address or address range in the connection filter policy. For instructions, see Configure connection filtering.

      Use this setting if the sending domain does not use email authentication. Be as restrictive as possible when it comes to the source IP addresses in the IP Allow List. We recommend an IP address range of /24 or less (less is better). Do not use IP address ranges that belong to consumer services (for example, outlook.com) or shared infrastructures.

    Important

    • Never configure mail flow rules with only the sender domain as the condition to skip spam filtering. Doing so will significantly increase the likelihood that attackers can spoof the sending domain (or impersonate the full email address), skip all spam filtering, and skip sender authentication checks so the message will arrive in the recipient's Inbox.

    • Do not use domains you own (also known as accepted domains) or popular domains (for example, microsoft.com) as conditions in mail flow rules. Doing so is considered high risk because it creates opportunities for attackers to send email that would otherwise be filtered.

    • If you allow an IP address that's behind a network address translation (NAT) gateway, you need to know the servers that are involved in the NAT pool in order to know the scope of your IP Allow List. IP addresses and NAT participants can change. You need to periodically check your IP Allow List entries as part of your standard maintenance procedures.

  3. Optional conditions:

    • The sender > is internal/external > Outside the organization: This condition is implicit, but it's OK to use it to account for on-premises email servers that might not be correctly configured.
    • The subject or body > subject or body includes any of these words > <keywords>: If you can further restrict the messages by keywords or phrases in the subject line or message body, you can use those words as a condition.
  4. Action: Configure both of these actions in the rule:

    1. Modify the message properties > set the spam confidence level (SCL) > Bypass spam filtering.

    2. Modify the message properties > set a message header: Set the message header <CustomHeaderName> to the value <CustomHeaderValue>.

      For example, X-ETR: Bypass spam filtering for authenticated sender 'contoso.com'. If you have more than one domain in the rule, you can customize the header text as appropriate.

      When a message skips spam filtering due to a mail flow rule, the value SFV:SKN value is stamped in the X-Forefront-Antispam-Report header. If the message is from a source that's on the IP Allow List, the value IPV:CAL is also added. These values can help you with troubleshooting.

      The Mail flow rule settings in the EAC for bypassing spam filtering

Use Outlook Safe Senders

Caution

This method creates a high risk of attackers successfully delivering email to the Inbox that would otherwise be filtered; however, if a message from an entry in the user's Safe Senders or Safe Domains lists is determined to be malware or high confidence phishing, the message will be filtered.

Instead of an organizational setting, users or admins can add the sender email addresses to the Safe Senders list in the mailbox. For instructions, see Configure junk email settings on Exchange Online mailboxes in Office 365. Safe Senders list entries in the mailbox affect that mailbox only.

This method is not desirable in most situations since senders will bypass parts of the filtering stack. Although you trust the sender, the sender can still be compromised and send malicious content. You should let our filters check every message and then report the false positive/negative to Microsoft if we got it wrong. Bypassing the filtering stack also interferes with zero-hour auto purge (ZAP).

When messages skip spam filtering due to entries in a user's Safe Senders list, the X-Forefront-Antispam-Report header field will contain the value SFV:SFE, which indicates that filtering for spam, spoof, and phishing (not high confidence phishing) was bypassed.

Notes:

  • In Exchange Online, whether entries in the Safe Senders list work or don't work depends on the verdict and action in the policy that identified the message:
    • Move messages to Junk Email folder: Domain entries and sender email address entries are honored. Messages from those senders are not moved to the Junk Email folder.
    • Quarantine: Domain entries are not honored (messages from those senders are quarantined). Email address entries are honored (messages from those senders are not quarantined) if either of the following statements are true:
      • The message is not identified as malware or high confidence phishing (malware and high confidence phishing messages are quarantined).
      • The email address is not also in a block entry in the Tenant Allow/Block List (messages from those senders will be quarantined).
  • Entries for blocked senders and blocked domains are honored (messages from those senders are moved to the Junk Email folder). Safe mailing list settings are ignored.

Use the IP Allow List

Caution

Without additional verification like mail flow rules, email from sources in the IP Allow List skips spam filtering and sender authentication (SPF, DKIM, DMARC) checks. This result creates a high risk of attackers successfully delivering email to the Inbox that would otherwise be filtered; however, if a message from an entry in the IP Allow List is determined to be malware or high confidence phishing, the message will be filtered.

The next best option is to add the source email server or servers to the IP Allow List in the connection filter policy. For details, see Configure connection filtering in EOP.

Notes:

  • It's important that you keep the number of allowed IP addresses to a minimum, so avoid using entire IP address ranges whenever possible.
  • Do not use IP address ranges that belong to consumer services (for example, outlook.com) or shared infrastructures.
  • Regularly review the entries in the IP Allow List and remove the entries that you no longer need.

Use allowed sender lists or allowed domain lists

Caution

This method creates a high risk of attackers successfully delivering email to the Inbox that would otherwise be filtered; however, if a message from an entry in the allowed senders or allowed domains lists is determined to be malware or high confidence phishing, the message will be filtered.

Do not use popular domains (for example, microsoft.com) in allowed domain lists.

The least desirable option is to use the allowed sender list or allowed domain list in anti-spam policies. You should avoid this option if at all possible because senders bypass all spam, spoof, phishing protection (except high confidence phishing), and sender authentication (SPF, DKIM, DMARC). This method is best used for temporary testing only. The detailed steps can be found in Configure anti-spam policies in EOP topic.

The maximum limit for these lists is approximately 1000 entries; although, you will only be able to enter 30 entries into the portal. You must use PowerShell to add more than 30 entries.

Note

As of September 2022, if an allowed sender, domain, or subdomain is in an accepted domain in your organization, that sender, domain, or subdomain must pass email authentication checks in order to skip anti-spam filtering.

Considerations for bulk email

A standard SMTP email message consists of a message envelope and message content. The message envelope contains information that's required for transmitting and delivering the message between SMTP servers. The message content contains message header fields (collectively called the message header) and the message body. The message envelope is described in RFC 5321, and the message header is described in RFC 5322. Recipients never see the actual message envelope because it's generated by the message transmission process, and it isn't actually part of the message.

  • The 5321.MailFrom address (also known as the MAIL FROM address, P1 sender, or envelope sender) is the email address that's used in the SMTP transmission of the message. This email address is typically recorded in the Return-Path header field in the message header (although it's possible for the sender to designate a different Return-Path email address). If the message can't be delivered, it's the recipient for the non-delivery report (also known as an NDR or bounce message).
  • The 5322.From (also known as the From address or P2 sender) is the email address in the From header field, and is the sender's email address that's displayed in email clients.

Frequently, the 5321.MailFrom and 5322.From addresses are the same (person-to-person communication). However, when email is sent on behalf of someone else, the addresses can be different. This happens most often for bulk email messages.

For example, suppose that Blue Yonder Airlines has hired Margie's Travel to send advertising email messages. The message you receive in your Inbox has the following properties:

  • The 5321.MailFrom address is blueyonder.airlines@margiestravel.com.
  • The 5322.From address is blueyonder@news.blueyonderairlines.com, which is what you'll see in Outlook.

Safe sender lists and safe domain lists in anti-spam policies in EOP inspect only the 5322.From addresses. This behavior is similar to Outlook Safe Senders that use the 5322.From address.

To prevent this message from being filtered, you can take the following steps:

  • Add blueyonder@news.blueyonderairlines.com (the 5322.From address) as an Outlook Safe Sender.
  • Use a mail flow rule with a condition that looks for messages from blueyonder@news.blueyonderairlines.com (the 5322.From address), blueyonder.airlines@margiestravel.com (the 5321.MailFrom address), or both.