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Port 25 is the standard TCP port used to send e-mail via the SMTP protocol. As a precautionary measure to stop spam, many ISPs are no longer allowing outbound traffic on port 25. The only way to send outbound e-mail is through the ISP's own SMTP host (also known as a smarthost), which may require the address in an e-mail's 'from:' line use the ISP's domain name. For instance, if you're using bellsouth.net as your ISP, the e-mail address would have to read email@example.com.
If users report that they can suddenly no longer send e-mail from their home systems, this is probably the reason why. This issue will probably affect desktop users far more than others, since most of the blocking is taking place in networks that host individuals rather than IPs for companies.
On the one hand, this has made it more difficult to casually use one's desktop machine as a "hit-and-run" spam sender with a disposable IP address. On the other hand, it also means a great many people who have custom domains will be inconvenienced.
Anyone who handles their own e-mail through a third-party mail system will only be able to receive -- but not send -- e-mail for that domain. For instance, if you have a Web site named thisdomain.com, and have both a POP3 mailbox and an SMTP sender hosted at that address, you'll be able to receive e-mail from thisdomain.com (since POP3 uses port 110), but won't be able to send e-mail with the sender's name as firstname.lastname@example.org.
Some people might try to work around this by sending mail for thisdomain.com through their ISP's smarthost, but the smarthost will typically reject any outgoing e-mails that don't have the same sender domain name as the ISP. (The exact name of the smarthost varies depending on the ISP; sometimes it can be as simple as mail.yourhostname.com on port 25.)
There are a couple of possible solutions. If you are working with an independently-hosted domain with its own e-mail servers, ask if they provide SMTPS, a secure variant of SMTP that requires user authentication and runs on port 465. Another possibility (if this isn't already being done) is to set up a VPN from each user's desktop to a mail host on a network that does not have port 25 blocked.
About the author: Serdar Yegulalp is editor of the Windows 2000 Power Users Newsletter and a regular contributor to SearchExchange.com.
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I ran into the same problem with my Cox account. I was using Thunderbird as my e-mail client. My "From" address is the "cox.net" account, but I added a "Reply to" address using my personal domain account "scottjames.biz." That way, when people I send messages to hit "Reply," it ends up going to my scottjames.biz account -- even though I sent the message from my "cox.net" account.
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This was first published in February 2005