Some Internet providers have started blocking port 25 in an attempt to help prevent direct to MX spamming, and abuse of open proxies and relays from their dialup and broadband customers.

This causes email clients not to be able to contact the hosting account mail server directly to send outbound emails. Receiving emails, though, works fine.

To get around this, customers are having to put their ISP's mail server name into their mail client settings on their pc so the mail can then be forwarded from the ISP's mail server to their hosting account's mail server and on to the recipient.  The email still shows it's from the correct sender's mail address - not the ISP's domain name.

Here is a list of some of the major ISPs that do this and their mail server names:

If you have ISP service from one of these providers, most likely you will not be able to use the SMTP service provided with your hosting account (this includes all hosting providers) and you must use the mail server of your ISP.

If you're not able to send mail using one of these servers (but you receive mail just fine), contact your ISP and find out if they are blocking port 25 and what mail server you should use.