Help. Unable to send via SMTP

[thernes]

All of a sudden none of the users on my cpanel boxes can send e-mail using their mail clients via SMTP. No problems receiving, but no go on the outgoing mail. Everything works just fine using the webmail. No changes have been done on the server lately, except for the automated cpanel updates. CFS installed and set up to allow connections on port 25. Just to make sure this was not a firewall issue, the firewall was disabled and no difference.
Looks like the connection can not be made properly, as no new entires show up in /var/log/maillog when I try sending.

Need some help solving this one and a couple of ideas on where I should start to look.

Edit to add: No, my ISP does not block port 25

[thernes]

Just gonna add some more info here: Everything works just fine when I set exim to also listen to port 26. When I try to telnet to port 25 on my boxes, the connection fails, but no problem connection to other smtp servers (like my ISP's) on port 25.

[cPanelDavidG]

Just gonna add some more info here: Everything works just fine when I set exim to also listen to port 26. When I try to telnet to port 25 on my boxes, the connection fails, but no problem connection to other smtp servers (like my ISP's) on port 25.

Note that ISPs will allow port 25 connections to their own mail server. It's when it's not their mail server that port 25 traffic tends to be blocked.

[4u123]

We are also experiencing timeouts only for the last couple of days.

My server monitoring is showing timeouts and we are getting complaints from customers that they cant connect when sending.

This seems ot be affecting about 8 of our servers. We havent made any changes at all.

Have there been some exim changes in the last couple of days in release ?

We are on 11.15.0-R18264

[psychodreams]

same issue here I also noticed when was port 26 taken out of tweak settings in server configuration

[jarodlwk]

you need to check with your customer if they are using an ISP that blocks port 25. I have the same issue but it turns out the end users' ISP blocks port 25 to combat spam. We had to enable port 26 for these users to send mails.

[4u123]

For me it was a DNS resolution issue causing the problem.