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I have my own mailserver, it used to be hosted by Hostgator but now I have a VPS.

From my home location I can send and receive mails as much as I want. It even works on my phone.

My other users also are able to login to their accounts when they're at my house, on my connection.

However: when they go home they can't. And I have no idea why.

So, TL;DR: When my friend sends a mail from his home thunderbird throws an error, when he does it from here (on his laptop) it works fine. The server is in England.

I know this isn't much to go on, but I have no clue where to start on this. I haven't set any host specific filters.

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Telling us the error message that he's seeing on the laptop would be useful. –  Evan Anderson Jan 24 '10 at 0:56
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2 Answers

up vote 2 down vote accepted

It's quite possible that your users' ISP is more draconian than yours. Often, ISPs will block all outbound SMTP traffic that is not aimed at their own SMTP farm. Have them look into their ISP's traffic policies; something may be posted on their website or at least mentioned in a forum thread on DSLReports.com. I'd tell you to have them call their ISP, however it's easier to juggle ferrets than get coherent information out of an ISP's first two or three tiers of "technical support".

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Port 25 is often blocked by consumer ISP's (both inbound and outbound). I remember a particularly frustrating conversation on the phone with tech support trying to get my port 25 unblocked. The support person kept repeating "we don't block ports" like a mantra. As if repeating it somehow made it true. I just gave up and started using different ports.

If it's just your friend's ISP blocking outbound port 25 and/or your ISP blocking inbound port 25, you can configure your mail server to listen on port 587 (the "submission" port) which is typical for mail-client smtp connections.

If your ISP is blocking outbound port 25, you'll have to have a third-party MTA that you can connect to and forward mail through. If you have an email account at a University, you can use those smtp creds (assuming they are using port 587).

If your ISP is blocking inbound port 25 and you want to have a real mail server (that other mail servers will deliver to) you'll have to have a third-party mail server somewhere that can listen on port 25 and relay the mail to you.

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I don't know if I agree that you'll need to find a third-party email server to pull this off but jdizzle is correct about the ports. I set up a corportate email server that I configured to use port 10025 because half our our users were blocked at home with the default port 25. –  Patrick R Jan 24 '10 at 1:11
    
Depending on what he wants maybe not. If he wants to receive real mail for a domain from other mail servers and his inbound port 25 is blocked, then he will. –  jdizzle Jan 24 '10 at 1:25
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