IMAP, SMTP traffic problem

[ds]

Hi,

in my small company we have pfSense 2.3.4-RELEASE-p1 as a gateway for all 20 workstations (one LAN). Outside of that network we have a new mail server (IMAP 143, SMTP 587).
For some reason we have strange situation that sometimes our mail clients in the LAN can not connect to the mail server (connection timeout in Thunderbird). Generally it is observed when more people are working/during the day. In the evenings/nights it seems to be ok. That problem is not observed when clients are using any other network provider, f.e. mobile network. Therefore, I wonder if there is anything I should configure on pfSense to have it working? Can it block mail services? Previously our old mail server was configured to use POP3 995 and SMTP 25 and it was working without any problem with the same pfSense.

Please advise, I have no idea how to solve it.

EDIT:
Attached is what I capture on client workstation when the problem occurs.

[JKnott]

Many ISPs block port 25, but allow a different one.  I use 465 for SMTPS and 993 for IMAPS

[ds]

Yes, it is one of the reasons why we decided to install the new server. It was problem that 25 was blocked in some other places, but none of the ports I use is blocked by our office ISP.

[KOM]

Those spurious retransmission and Dup ACK errors are going to hang your connection and cause the issues you are seeing.  Do you see those errors for any other communications, or just with this mail server?