dev.null

Well-Known Member
May 27, 2003
85
2
158
I honestly think this is on the remote end, I'm posting this here to see if you guys have any suggestions to offer. The only thing I think that could be doing this on my cpanel box is one of the security settings, but all the ones I look at don't seem to be related.

Is there anything in cpanel that blocks network connections at the application layer?

I have a client that sends mass email through smtp.com. They have a php written that pulls email from their queue (database) and connects to smtp.com for delivery.

After working for years, it quit working starting last week. Basically the 3-way tcp handshake happens, then nothing. In less than a minute the remote end (smtp.com) issues a disconnect.

They've offered a number of different ports to use, none of them work. My cpanel has several IPs, so I switched to use one of the other IPs to transmit from, it began working again.

12 hours later, the second IP also quit working.

When I telnet to smtp.com from my personal computer, I see the 3-way handshake and immediately their server sends a 220 ESMTP message, and then awaits commands.

From my cpanel I never get the 220 packet. So I think the network layer is working fine, but the application layer isn't working - like their MTA has blacklisted me, allowing the tcp connection but then ignoring it.

I've checked with my provider (linode) to make sure they don't have some anti-spam appliance that is doing this, they have nothing.

I've looked through my cpanel's iptables rules, but don't see anything there. I even explicitly allow their IP at the top of my INPUT and OUTPUT chains - not that it matters since I'm actually seeing the packets traverse netfilter.

When I ask smtp.com about it they telnet to their server and say "see, works for me!".

You guys have any ideas?
 

24x7server

Well-Known Member
Apr 17, 2013
1,911
96
78
India
cPanel Access Level
Root Administrator
Twitter
Hi,

It does not appear to be a cPanel issue, but rather I suspect it to be a firewall issue. For an quick check, can you just disable the firewall, iptables, or whatever firewall you have on your server and then try to connect? May be some modification in the firewall blocked it.
 

cPanelMichael

Administrator
Staff member
Apr 11, 2011
47,913
2,203
363
Hello,

Can you browse to "WHM >> SMTP Restrictions" and verify if that option is enabled? Also, if you use the CSF firewall, can you verify if the "SMTP_BLOCK" setting is enabled?

Thank you.