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:

ISP	Outgoing Mail Server (SMTP)
Aliant / NbTel	smtp.nbnet.nb.ca, smtp.vibe.net, smtp.aibn.nb.ca
AT&T WorldNet	mailhost.worldnet.att.net
Charter	smtp.charter.net
Comcast/ATTBI	smtp.comcast.net
Cox.net	smtp.west.cox.net, smtp.east.cox.net
DCAnet	postoffice.dca.net
EarthLink	mail.earthlink.net
ISPWest	smtp.ispwest.com
Microsoft's MSN	smtp.email.msn.com
MindSpring	smtp.mindspring.com
NetZero	smtp.netzero.net
People PC	mail.peoplepc.com
Prodigy Internet	smtp.prodigy.net
SBC	smtp.wans.yahoo.com, smtp.swbell.yahoo.com, smtp.snet.yahoo.com, smtp.sbcglobal.yahoo.com, smtp.prodigy.yahoo.com, smtp.pacbell.yahoo.com, smtp.nvbell.yahoo.com, smtp.flash.yahoo.com, smtp.ameritech.yahoo.com, mail.sbcglobal.net
Sonic.net	mail.sonic.net See: http://www.sonic.net/support/faq/advanced/port_25.shtml
Sprynet	smtp.sprynet.com
Verio	mail.verio.net
Verizon/GTE (Dial-Up)	smtp.verizon.net, smtp.vzavenue.net

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.