Outlook 2019 cannot connect to IMAP server, while POP3 works with the same credentials

Question

Dear Microsoft Support Community,

we are an IT service provider and are currently troubleshooting an issue for a customer using Microsoft Outlook Professional 2019.

After migrating the customer's mailbox to a new mail server, Outlook 2019 is unable to configure the mailbox via IMAP.

Manual configuration:

Incoming server (IMAP): plesk.servcp.de Port: 993 Encryption: SSL/TLS Username: full email address Password: current mailbox password

Outlook reports that it cannot log on to the incoming IMAP server and asks us to verify the email address and password.

The important point is that POP3 works immediately with exactly the same email address and password. Webmail also works correctly. Therefore, the mailbox credentials themselves are valid.

We have already:

• configured the account manually
• checked Windows Credential Manager
• verified that no credentials for plesk.servcp.de are stored there
• tested the same credentials successfully with POP3

We would like to know whether Outlook 2019 has any known issue with IMAP authentication after changing mail servers.

In particular:

1. Does Outlook 2019 cache old IMAP or Autodiscover information somewhere outside Windows Credential Manager?
2. Is creating a completely new Outlook profile recommended in this situation?
3. Should Secure Password Authentication (SPA) be disabled?
4. Are there specific TLS or authentication requirements for Outlook 2019 when using IMAP on port 993?
5. Is there an Outlook diagnostic log that shows whether the failure occurs during TLS negotiation or IMAP authentication?

The mail server is a standard external IMAP/SMTP server based on Plesk/Dovecot, not Exchange or Outlook.com.

Thank you very much for your assistance.

Kind regards, Andreas FritzDear Microsoft Support Community,

we are an IT service provider and are currently troubleshooting an issue for a customer using Microsoft Outlook Professional 2019.

After migrating the customer's mailbox to a new mail server, Outlook 2019 is unable to configure the mailbox via IMAP.

Manual configuration:

Incoming server (IMAP): plesk.servcp.de
Port: 993
Encryption: SSL/TLS
Username: full email address
Password: current mailbox password

Outlook reports that it cannot log on to the incoming IMAP server and asks us to verify the email address and password.

The important point is that POP3 works immediately with exactly the same email address and password. Webmail also works correctly. Therefore, the mailbox credentials themselves are valid.

We have already:

• configured the account manually
• checked Windows Credential Manager
• verified that no credentials for plesk.servcp.de are stored there
• tested the same credentials successfully with POP3

We would like to know whether Outlook 2019 has any known issue with IMAP authentication after changing mail servers.

In particular:

1. Does Outlook 2019 cache old IMAP or Autodiscover information somewhere outside Windows Credential Manager?
2. Is creating a completely new Outlook profile recommended in this situation?
3. Should Secure Password Authentication (SPA) be disabled?
4. Are there specific TLS or authentication requirements for Outlook 2019 when using IMAP on port 993?
5. Is there an Outlook diagnostic log that shows whether the failure occurs during TLS negotiation or IMAP authentication?

The mail server is a standard external IMAP/SMTP server based on Plesk/Dovecot, not Exchange or Outlook.com.

Thank you very much for your assistance.

Kind regards,
Andreas Fritz

Answer 1

Supported Outlook-side causes and actions for this scenario are:

1. Check for an IMAP app-password requirement Microsoft documents that Outlook IMAP/POP connection failures such as server interruption or unexpected termination can be caused by an invalid app password. If the mailbox provider uses app passwords, create a new one in the provider’s web security settings, then update it in Outlook:
   • Outlook > File
   • Select the affected IMAP account under Account Information
   • Account Settings > Server Settings
   • Paste the new app password in Password
   • Select Next > Done
2. Check for a password-character issue specific to Outlook IMAP Outlook 2016 / Outlook 2019 / Outlook for Microsoft 365 can fail IMAP authentication if the password contains Unicode characters such as ä, ö, or ü. If the mailbox password contains Unicode characters, change it to a password without Unicode characters and test IMAP again.
3. Check TLS 1.3 on Windows 10 Microsoft documents a known Outlook IMAP/POP issue where TLS 1.3 on Windows 10 can cause connection failures. Windows 11 TLS 1.3 is documented as working fine with Outlook. If the affected machine is Windows 10 and TLS 1.3 is configured, remove the TLS 1.3 registry keys under: Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols Risk: registry changes can affect system security and connectivity. Export the existing Protocols key first so the change can be reverted if needed. After removing TLS 1.3, restart Outlook, and if sync still does not start, restart Windows.
4. For Outlook.com accounts only, authorize the IMAP session If the mailbox were Outlook.com configured as IMAP in multiple clients, Microsoft’s documented workaround is to approve the recent IMAP sign-in on the Recent activity page. This does not apply to a standard external Plesk/Dovecot server.

For the specific questions:

• Cached old IMAP or Autodiscover information outside Credential Manager: no supported information was found in the provided material.
• Creating a completely new Outlook profile: no official Microsoft documentation in the provided material confirms this as the recommended fix for this exact scenario.
• Whether SPA should be disabled: no supported information was found in the provided material.
• Specific TLS or authentication requirements for Outlook 2019 IMAP on 993: the supported items in the provided material are the TLS 1.3 issue on Windows 10, possible app-password requirement, and the Unicode-password limitation.
• Diagnostic logging to distinguish TLS negotiation vs IMAP authentication: no supported information was found in the provided material.

Because POP3 and webmail already work with the same mailbox, the most directly supported Outlook-specific checks are:

• test with a newly generated app password if the provider supports or requires it
• verify the mailbox password does not contain Unicode characters
• if the client is Windows 10, inspect whether TLS 1.3 is configured and remove it only after exporting the registry key first

---

• Outlook IMAP or POP server unexpectedly terminated the connection, and the server was interrupted
• Authentication fails when you use an IMAP server in Outlook 2016