Getting 421 Server error when telneting into port 25

[Brazen]

Ok, so I'm trying to figure out why email is not going through, if that wasn't obvious.

So from our email server, I try telneting into their email server with no luck. I think maybe it is a problem with our server, so I open up outgoing port 25 on the firewall from my laptop and try it from there with the same result. I try telneting into some other email server out on the internet and it works just fine.

This may lead you to believe it's their server having a problem, BUT I take my laptop off our network and plug it directly into the internet with a spare public IP address (for just such occasions) and try telnetting to their email server gain, and IT CONNECTS!

So the problem is only between our main network and their server. Any ideas?

Edit: I also just checked our firewall logs and it did not have anything about it blocking the connection. So I don't think our firewall is the culprit for any odd reason.

 

[TheSiege]

probably your ISP or a firewall, im willing to bet its the ISP

 

[DaiShan]

Have you asked the remote admin to check their firewall rules? It's very easy to block access to a port for a single IP address in something like IP tables, in fact I use a script to automatically drop packets destined for port 22 by abusers (those who fail on login more than 5 times) based on their IP address. It really sounds like they are doing something similar and you are caught up in some sort of blacklist. If that doesn't yield any results, feel free to PM me with any relevant information and I'd be happy to take a look (really just ip addy of their mail server)

 

[Brazen]

Originally posted by: TheSiege
probably your ISP or a firewall, im willing to bet its the ISP

How could it be my ISP? 😕 Not that I'm saying you are wrong, I just don't know how my ISP could be causing this.

Originally posted by: DaiShan
Have you asked the remote admin to check their firewall rules? It's very easy to block access to a port for a single IP address in something like IP tables, in fact I use a script to automatically drop packets destined for port 22 by abusers (those who fail on login more than 5 times) based on their IP address. It really sounds like they are doing something similar and you are caught up in some sort of blacklist. If that doesn't yield any results, feel free to PM me with any relevant information and I'd be happy to take a look (really just ip addy of their mail server)

This is my guess which sucks because it means I'm at the mercy of the competence of the other server's administrator(s). I tried getting contact information for their admin yesterday and I'm still waiting.

 

[RebateMonger]

If you can Telnet on TCP Port 25 to other mailservers, but can't Telnet to to this particular server, then that server is blocking your IP.

 

[spidey07]

Originally posted by: RebateMonger
If you can Telnet on TCP Port 25 to other mailservers, but can't Telnet to to this particular server, then that server is blocking your IP.

421 error means he's connecting to the smtp server and the server is telling him "i'm not gonna service you".

So it does seem like he's been blocked. I'm not a mail expert though.

 

[RebateMonger]

Originally posted by: spidey07
421 error means he's connecting to the smtp server and the server is telling him "i'm not gonna service you".

In the Exchange Server world, a 421 comes as:
"421 Cannot connect to SMTP server"

If Connection Filtering is turned on at the remote server (i.e. Block List), then the sender would normally get an email response that says, "Your connection was refused because you are on the Spamcop Block List" (or something equivalent to that). But I guess you can turn off the response, so he wouldn't get any email response at all. I always leave the response on, since most folks don't want to refuse messages from potential customers without telling them why.

My Exchange Servers typically refuse about half of the requested connections nowadays, because the sender's server is on one of the Block Lists that I use.

Either way that remote Server is refusing the connection from his Server's IP address.