My ISP (
Netvigator) goes to great lengths to block spam mails sending out from its users. All its users cannot connect to SMTP severs outside. This creates a problem for me as I host my own mail server at home.
Normally, when a mail server tries to deliver an email, it will connect to the destination mail server directly. e.g.
Draft an email to abc@yahoo.com on my PC --->
my mail server --->
yahoo's mail server
But as mentioned above, my ISP blocks all the access (port 25) to the outside world. So there is no way for my mail server to connect to other mail servers and deliver the emails.
Luckily, by the SMTP standard, I could force the mails route through my ISP's mail server:
Draft an email to abc@yahoo.com on my PC --->
my mail server --->
Netvigator's mail server --->
yahoo's mail server
So far so good. The remaining problem is: the Netvigator's SMTP server requires authentication which my mail server (
Qmail) doesn't support! Thanks to open source, it is easy to patch the code though.