A Question about resolving MX Records...

I have a program I've written which queries name servers for MX Records of domains, and then does the lookup of the A Record from the response. My question is this... if the mx lookup returns no results what should happen? Should I just query the A record for the given domain and try to send mail to that IP, or do I do nothing and consider it a bad domain? Whats RFC dictate? I cant really find anything about it.

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I don't know what the RFC states, but it is perfectly legitimate to not set an MX record for a domain. It may be bad practice, but the DNS record can have an empty MX record. Not every domain has to have an associated e-mail service/host. Sending mail to the A record in such circumstances would be pointless...

Not completely pointless.  Mail servers (such as sendmail) will attempt to send directly to a host name if it exists when it can't find an MX record.  In other words, it will do an A record lookup against the domain name.    See:  http://en.wikipedia.org/wiki/MX_record#MX_Record_and_spam under the section on MX Record and Spam.

Here's the RFC for reference:  http://tools.ietf.org/html/rfc2821

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Yes, I see the article suggests that not having an MX record but having an e-mail server on the A record host is a way of avoiding some SPAM. And you cite the RFC, that specifies reverting to the A record in the absence of MX records. Thanks for the correction.
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