Roundcube - Connection Error (Failed to reach server)

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

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)

                                                                         

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

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?

Also what version of PHP and OS are you running?

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

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

The email had a plain signature

Thanks
Firstname lastname

Removing the signature, email is going.

Changing password didnt help

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).

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).

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.

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