Hello all,

i m having a doubt, we are servicing for an ISP and if a customer configures his Outlook Express express for some other domain, with our ISP service, then he is not able to send mails. It has been told to us that customer will not be able to send mail becaue port 25 is blocked. What i want to know is whether it is port blocking or relay blocking?

Without further information, I can only speculate.

In order to fight spam sent by infected computers some ISPs have chosen to block port 25.

This can simply be done by a packet filter dropping all packets with the destination port 25, effectively blocking access to all other mail servers out there which are listening on this port by default.

It is possible that ISP let packets through to his own mail server, only. This way he controls what mail is going out. Your ISP might have been blacklisted as a spammer network and has taken this step to get delisted again.

It is a drastic measure, as it not only blocks access for spam, but also legitimate access to other mail service provider.