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I have a dedicated server that I plan to use to host multiple web applications for a few different clients. If I have client1.com and client2.com, I would like web apps on those domains to be able to send email from their respective static IP addresses.

But I recently learned this is not possible. I guess you can't pick which IP your email comes FROM. I want to be able to do that so when customers' mail servers perform spam screening, the IP the email came from will match up to client1.com or client2.com, whichever is applicable.

So if I can't pick which IP the email gets sent from, then I guess I need to pay for some service that will allow this.

So I could still have www.client1.com on my own server, but then pay for a service to host mail.client1.com, and send email with their SMTP server. I also need to offer my clients email accounts, so I may as well look for a service that does email both ways, right?

I hope I am making sense here. :)

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2 Answers

up vote 3 down vote accepted

Why does it matter so much that the IPs sending the mail match up with the domain it's coming from? It's extremely common for organizations to have dedicated SMTP servers that have wholly distinct IP addresses from the domains they claim to be sending mail from, or to even outsource the sending of mail from their apps altogether. Google's Gmail service is one such (very visible) example of the former, while anyone using a Google Apps domain (and sending via Google's SMTP server) is an example of the latter.

With a properly-configured SPF record on domain1.com and domain2.com, you could have those domains on IPs 1.1.1.1 and 2.2.2.2, but send e-mail from a server on IP 3.3.3.3, a server that could potentially have the domain name domain3.com, and no properly-configured mail server will reject it or flag it as spam (at least not due to the origin). (If you're worried about improperly-configured mail servers, don't -- nothing you can do about them, and they're so few anyway that's not worth worrying about anyway.) If you're worried about customer perception of seeing an e-mail purporting to be from domain1.com but having actually been sent by domain2.com, well, 99.9% of users will never know, and that 0.1% of users that do see that will know it doesn't mean anything.

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This is good news and if you're right I will mark this as the answer. :). I read somewhere that smtp servers, when receiving mail, would check the IP it came from and make sure the domain matched when doing a reverse lookup. If this is not true then I am worrying about nothing. –  Chris May 12 '11 at 0:10
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@Chris this is a VERY common mis-conception. The spam filtering does a reverse lookup on the HOST NAME sent by HELO/EHLO NOT the MAIL FROM: command. So as long as your mail server's actual, real host name has a proper PTR record you will be fine. –  Zypher May 12 '11 at 0:20
    
If you guys are in agreement then I am super happy. Thank you. =) –  Chris May 12 '11 at 0:26
    
Alternatively, use another SMTP server as a mail relay that accepts outgoing mail from those two servers. –  Matt May 12 '11 at 0:50
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MailChimp provides a hosted mail platform that seems pretty good. I haven't used it myself so can't vouch for them but they have a free trial available.

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