Some Internet Service Providers do not allow their customers to use other SMTP servers than their own on the default outgoing mail port. This is a part of their efforts to limit spam coming from their networks. These ISPs block the default outgoing mail port, which is 25. You can use our SMTP service on ports 587 or 465 instead.
An alternative solution is to configure your e-mail program to use your ISP's SMTP server for outgoing e-mail.