Cant send email anymore

Hello All, hope this is the correct forum for this question.

Windows / Mac network

I’ve been sending email with Mac’s and PC’s without a problem for years.
Email is received via pop3 and sent via SMTP port 25.
The network firewall is set to allow any LAN to WAN traffic to pass though.
This setup has worked fine for at least 2 years.

Within the last 2 weeks the Macs have stopped sending emails, still receiving mail but not sending mail.
Sending and receiving email from the PC’s works fine without any problems.

When I check the hardware firewall logs they say the SMTP traffic is passing through.
When I disable the hardware firewall, sending email from the mac work fine.

You would think the firewall would be at fault, although no settings have changed on the firewall to cause a problem. Emails sent from the macs up intill 2 weeks ago had no problem with the firewall.

Is it possible the mac’s would have downloaded a software /OS update to cause this problem?

We have tried setting up the email on a brand new mac straight from the box, the sending problem still occurred .

I have disabled the Mac software firewall.

Has anyone heard of a problem like this?

 

Cheers devilot, I've check the sending port a couple times, still I will give that a try. Thanks for the quick reply.