Issue utlook can’t download attachments or save drafts via IMAP (Plesk mail server)

[Johnny9977]

Server operating system version

Ubuntu 22.04.5 LTS

Plesk version and microupdate number

18.0.73 #1

Hi everyone,


I’m running into a strange issue with my Plesk mail server (Postfix + Dovecot) and Outlook (Mac & Windows) clients.

For the past few days, I have been unable to properly handle emails with attachments. The following problems occur:

• Attachments won’t download (“Attachments must be downloaded again from the server. Please try again later.”).
• Messages with attachments can’t be replied to or forwarded.
• Drafts can’t be saved (“Cannot save to Drafts folder”).
• Folder synchronization is extremely slow — it takes minutes to load message lists.

Webmail (SoGo) works perfectly fine, so IMAP itself seems functional.

However, in the server logs I see multiple lines like:

dovecot: imap-login: SSL_accept() failed: error:0A0000EB:SSL routines::no application protocol
dovecot: imap-login: SSL_accept() failed: error:0A0000C1:SSL routines::no shared cipher

and continuous brute-force login attempts such as:

plesk_saslauthd: failed mail authentication attempt for user 'randomname'

My current TLS setting is:

/etc/dovecot/conf.d/11-plesk-security-ssl.conf
ssl_min_protocol = TLSv1

Fail2Ban is already active.

Has anyone seen this behavior before?

Could the outdated TLS setting cause Outlook to repeatedly reconnect and fail when downloading attachments?

Should I raise the minimum protocol to TLSv1.2 or higher?

Or could this also be caused by Dovecot index corruption or overload from brute-force attempts?

Any hints or recommended configuration for stable IMAP performance with Outlook would be appreciated.


Thanks in advance!

 

[Sebahat.hadzhi]

Could the outdated TLS setting cause Outlook to repeatedly reconnect and fail when downloading attachments?

Hello, @Johnny9977 . If your email client still uses the TLS 1.1 protocol it is very like to be causing the issue as it is by default disabled in the Postfix and Dovecot configuration.

Should I raise the minimum protocol to TLSv1.2 or higher?

Yes, please do try changing the protocol. If that doesn't work, please provide us with the Outlook version you are using.