NimbostratusI don't think the MX records for your incoming SMTP servers have to match the IP-addresses that are used for systems that are sending mail. It's true that most SMTP servers will do a reverse lookup for systems that try to deliver them mail as one of the many checks to prevent accepting mail from a malicious source, but it has nothing to do with the MX records. These are only used for incoming mail.
Also check your SPF records if you are using them. Make sure these are matching the servers that are known to send mail for your domain.
