Intermittent connection error with an IMAP mailbox

Question

One of our customers is having an issue with new Outlook where after we set up their email as IMAP (which we host), after a while Outlook would no longer recognize the password. When they went into Settings, it showed a message says "Sign-in is required. Click Sign in to resolve." The customer did that, and the sign in failed, so they closed new Outlook and did something else for a while we investigated.

However, the user came back to the computer a little later, opened Outlook on a lark, and everything was back to normal and they could see all their email again. This has happened two days in a row, around 09:00 Central/16:00 UTC.

Does anyone have any idea of what might be causing that? The customer forwarded screenshots of the information they got when they clicked "Troubleshooting Details", but I don't know how helpful that will be for y'all.

Answer 1

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Hi TLRS-Sandwich

Please reply with only these details:

• Does webmail work during the same Sign-in is required period?
• Do the server logs show failed IMAP authentication at the same time?
• What IMAP port and encryption method are being used?
• Does the issue happen in Classic Outlook or Thunderbird, or only in New Outlook?
• Are there any rate-limit, firewall, TLS, or account-lock events around 09:00 Central / 16:00 UTC?

Please try the steps below:

1) Test outside New Outlook when the issue occurs

When the problem happens again, test the same mailbox immediately in:

• Webmail
• Another IMAP client, such as Classic Outlook or Thunderbird

If webmail and another IMAP client also fail, please check your IMAP server or authentication logs first. If they continue working while only New Outlook for Windows shows Sign-in is required, the issue is more likely related to the New Outlook account connection or cached sign-in state.

2) Check your hosted IMAP logs for the exact failure window

Since you host the IMAP service, check the mailbox’s IMAP authentication and connection logs around 09:00 Central / 16:00 UTC.

Look for:

• Failed password attempts
• TLS/SSL negotiation errors
• Temporary account lockout or rate limiting
• Connection limit errors
• Firewall, proxy, or geo/IP blocking events

3) Verify the current IMAP/SMTP requirements

Confirm that the account is using the exact current settings required by your hosted service, including:

• Server names
• Ports
• Encryption method
• Username format
• Authentication requirement

4) Check whether an app password or special mailbox password is required

If the mailbox uses two-step verification or any additional security layer, confirm whether your hosted service requires an app password or dedicated mail password for IMAP.

5) Remove and re-add the account only after server checks

If the server logs do not show a clear failure, remove and re-add the account in New Outlook.

Go to: Settings > Accounts > Your accounts

Then:

• Select the affected IMAP account.
• Choose Manage.
• Remove the account.
• Restart New Outlook.
• Add the account again using the current IMAP/SMTP settings.

6) Use Classic Outlook or webmail as a temporary workaround

If the server logs show no IMAP failure and the issue happens only in New Outlook, use Classic Outlook, webmail, or another IMAP client temporarily while collecting evidence.

This avoids repeated password changes when the mailbox itself may not be the problem.

7) Submit diagnostics from New Outlook

In New Outlook, go to: Help > Support

Then select the affected account and submit the issue with redacted details, including:

• Exact time of failure
• Account type
• IMAP/SMTP ports and encryption
• Whether webmail worked at the same time
• Whether Classic Outlook or Thunderbird worked at the same time
• Any related server-log findings, with sensitive data removed

Please tell me how it goes. If the problem persists, please don’t hesitate to reply to this thread and I will be happy to help. Have a lovely day!  

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