IMAP connecting not working on Windows, does on mobile and MAC

[PieterVH]

Hi There,

My old server got corrupted after a restart (all was working before), so I took a new dedicated server to move all my 96 accounts. Server managers dit that for me. So far all good. Now people with windows computers start complaining the mail can't connect (tried with Thunderbird, and Outlook) with their mail accounts.

Mac mail: connects within a second
Outlook Mac: connects within a second
Outlook iPhone: connects within a second.
Webmail: works like a charm.

DNS records are good (e-mail deliverabillity tab gives a checkmark)
I restarted IMAP server and mailing server several times.

My server guys say nothing is odd and it can't be something in the new WHM configuration. I thought of IP bans, but the logs are clean. Is there anyone who has a clue of what might be happening here?

 

[kodeslogic]

Try to change the incoming server settings as below:

Port: 993 or 143
Connection Security: Auto

 

[cPRex]

You could also try disabling the firewall temporarily on the new machine to see if that is related to the issue.

 

[ITHKBO]

It sounds like it might be a issue related to TLS protocol versions active on the Windows clients *if the firewall and cPHulk Bruteforce Protection is not showing anything*.

Can you crosscheck what TLS and ciphers are allowed on the server in EXIM and one of the problematic clients?
Exim Configuration Manager-> Basic Editor -> Security -> Options for Open SSL
Should read something like this: +no_sslv2 +no_sslv3 +no_tlsv1 +no_tlsv1_1

To check on Windows we use IIS Crypto as powershell is a pain with this kind of audit.

Nartac Software - IIS Crypto

www.nartac.com

Always be wary with executables and do your own research for safety.
I have done a quickscan for you with the latest version

VirusTotal

VirusTotal

www.virustotal.com

We have had past cases with Windows Message User Agents (MUA's) trying the connection with a lower version than allowed by our servers causing connection issues without a clear error in MUA. This in turn required us to monitor live port access on the mail server. After some time we found out some Windows clients used TLS1.0 or 1.1 to connect which are considered End of Life (EOL) disabling the protocols on the Windows client with a restart solved that issue.

Be sure to verify that no legacy software still requires any EOL protocol before disabling its functionality.

 

[martin MHC]

We have a very similar issue this morning; it appears that a MS Outlook update (I assume) has caused Outlook to somehow mess up displaying data to the customer. We have logs on the server that emails are received by the mailbox and that Outlook connects ok to the server, but Outlook internally doesn't display these emails to the client.