Outbound mail, SMTP Port 25, Blocked

It seems that some ISPs (Internet Service Providers) such as Wanadoo and AOL, are blocking port 25 traffic to all but their own smtp servers, in an attempt to combat spam.  The idea is that viruses on infected customers computers connect to open smtp servers on port 25 and mail out junk.  This seems like a rather clunky way for an ISP to try to deal with spam, they are effectively cutting out part of their support for TCP/IP - the connection they are supposed to provide.

Spam mail is huge problem, with current spam filters struggling to avoid marking legitimate email as junk - known as false positives in the industry. This means that real email is constantly ending up in peoples junk folders. So at least the ISPs are trying to help.

Our heavyweight mail service Tonsho, which allows sending of large email attachments and special spam filtering, is affected by this port blocking. We need users to be able to connect to our smtp server. Therefore we have also made the Tonsho smtp server available on port 26.

Our spam protection system is based on authenticating users, and only accepting mail from authenticated users.  In the first instance we distinguish between automated emailers and genuine humans by means of a captcha.

We maintain a list of allowed contacts.  If a user who is not in your allowed contacts list send you an emails, they are automatically sent a challenge mail which links to a captcha. Once users have solved the captcha and been added to your allowed contacts, mails come straight through. Meaning they only have to authenticate once.

If you want to try Tonsho for yourself, for free, you can sign up here.