Ensuring that email gets through

We have a program that has the capability of sending and receiving email.  Consider that many users are connecting through an ISP to the internet.  The problem that we sometimes encounter is that we are sending an email directly through to our SMTP server and the ISP does not like that, for obvious reasons.  I believe the process goes as follows:
1. user sends email to xyz@abc.com using mailserver.ourdomain.com
2.  traffic gets to ISP, ISP checks that the email is not using their SMTP server and sometimes drops this request

We do not want the user to bother with finding out their login info and ISP SMTP server address, that is not an option.  So far, we only have work arounds to this and would like to know what options we have to do this.  For instance, http traffic is never dropped so perhaps there is a way of carrying the email inside an http request.  One more thing to note is that webmail per se is not an option, there are/will be too many users so an apache/imap/webmail solution is not an option.
Any suggestions are welcome.  
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When you say mailserver.ourdomain.com do you mean you actually have a mailserver which the program sends them to (i.e. exchange, sendmail) or does the program attempt to deliver the email directly to the recipient domain bypassing your ISP's smtp server altogether?

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Hi bisonfur37,
Well, you can change the ports your server and the clients are using. This sounds like a very bothersome ISP.


As kain21 points out, there is no general requirement that your mailserver relay mail through the ISP's mailserver. In fact, depending on the rules your ISP uses, that may be against policy.

Some ISP's, such as AOL, implement their mail handling by proxying SMTP, which can cause some strangeness if you are not expecting it. In general, unless the terms of service explicitly state that you cannot use SMTP directly to the destination, there should be no need to use the ISP's server at all.

I hope that the above is helpful.

- Bob (aka RLGSC)
I would agree with NetworkArchitek, but if your firewall/internet router does PNAT then you only need to change the port SMTP uses at the client end. Let's say you make it 8080, just port translate incoming connections to your mail server 8080 to 25.
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