The socket error, 10051, translates to ‘Network is unreachable.’ This is often the case if your ISP blocks sending mail (blocking port 25) through anyone other than their OWN smtp server. They do this to foil spammers, and it is becoming more and more common.
Some Internet providers, (particularly PeoplePC, AT+T, and Earthlink) have recently started blocking port 25 in an attempt to help prevent unsolicited e-mail from outside sources being sent through their servers.
Unfortunately a side-effect of this practice is that it prevents users from accessing our SMTP servers. If you are getting error messages when you try sending mail which state that smtp.mrhost.ca is not responding or not found, contact your ISP, and find out if they’re blocking port 25. If they are, talk to them about it. They may be willing to remove the block for you. If they are not, you have two options:
Firstly, you can use your ISP’s SMTP server to send mail instead of ours. A lot of the time this will solve your problem.
Sometimes, however, an ISP will not allow e-mail to be sent through their SMTP server with a different domain. In other words, you may only be able to use their SMTP server if you have your reply address set to your ISP-based e-mail address, not your custom domain name address that you have at Mr.Host. If this is the case, once again talk to your ISP. They may be able to make exceptions for your addresses or your domains.
Secondly, you can try modifying the server port number for the outgoing mail server (usually under the Advanced settings of your mail settings), and change it from 25 to 2500 (or 587 or 8080). Mr.Host runs SMTP service on ports 25, 587, 2500 and 8080.
The only other option is to use a different Internet Access Provider that will not block port 25. 🙁