| As a method of spam prevention, should I bounce email that comes from servers that are not the MX of the senders domain? |
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The quick and short answer is no.
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The inbound and outbound SMTP servers for any given site's domains don't necessarily (and indeed don't usually) reside on the same server. Indeed, most larger companies and ISP's split their inbound and outbound servers....and as such would break the rule.
With this said, it would be quite possible to publish an MX record for the reverse DNS of the SMTP client which points to the SMTP client. Aparently, 50% of the net is configured this way.
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