Roundcube - Connection Error (Failed to reach server)

[anandmys]

Logged in to user control panel. Use auto login to roundcube

Email 1 - Try sending email I get the error  "Connection Error (Failed to reach server)"

Other 9 email ids works flawlessly

Please suggest

[anandmys]

Error logs say

[05-Dec-2025 11:05:13 +0000]: <g9kaaxsd> IMAP Error: Login failed for user@dom                                                                             
ain.tld against localhost from 127.0.0.1 (X-Real-IP: 59.93.x.y,X-Forwarded-F                                                                             
or: 59.93.x.y, 59.93.x.y). AUTHENTICATE PLAIN: Authentication failed. in /                                                                             
usr/local/cwpsrv/var/services/roundcube/program/lib/Roundcube/rcube_imap.php on   
line 211 (POST /?_task=login&_action=login)

[Starburst]

Make sure you have the IMAP option enabled when you built PHP.

[overseer]

If it's automated login from the panel, should be using an API key to authenticate. Could be an odd gremlin. Try logging out/logging back in. Or could be some specific problem with the account. Can you test logging in manually? Are you able to reset the user credentials without interrupting customer access?

[Starburst]

Also what version of PHP and OS are you running?

[anandmys]

Strangely, this is happening to only 1 email id out of 100s of emails across multiple domain names.

[cyberspace]

Try to reset the password for the problematic mailbox. Don't use the chars like "!@#%^&" because it could cause problems in some cases.

[anandmys]

The email had a plain signature

Thanks
Firstname lastname

Removing the signature, email is going.

Changing password didnt help

[6Sense]

Try again with your signature & check your Mod security logs, might need to disable one or two rules.

From CWP menu

Security > Mod Security & cick on logs

I had similar issue ages ago & Mod security was denying emails from Roundcube that had anythig more than a few words in the email (was only with Roundcube, all mail from other mail clients worked fine).

[anandmys]

Thank you. It was mod security issue.

There were issues with mod security paths and the file where I was adding the exception rules

Try again with your signature & check your Mod security logs, might need to disable one or two rules.

From CWP menu

Security > Mod Security & cick on logs

I had similar issue ages ago & Mod security was denying emails from Roundcube that had anythig more than a few words in the email (was only with Roundcube, all mail from other mail clients worked fine).

[Starburst]

If you are running OWASP CRS you can disable the global rule IDs 981176 and 949110.

Or add the following to your whitelist before the rules load:

SecRule REQUEST_URI "@contains /roundcube/" "id:{some number},phase:1,nolog,allow"
SecRule REQUEST_URI "@contains /roundcubemail-*/" "id:{some number},phase:1,nolog,allow"

Also if you send files you might have to disable certain file extension rules.

[Starburst]

NOTE: The {some number} mentioned above would use whatever number sequence you are using for custom rules.