You might want to run your MTA's ip address through a blacklist checker.
Also, do you use srv records? Has anything here changed?
I've found that when it comes to email, its quite plausible for your system to break because of an external party, for example, do you use the relay of your ISP? If you do, run their ip through a blacklist db, in case someone on their end has been spamming.
If you didn't change anything, then consider the above factors, I hope it helps :)
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Kind Regards,
Mr Gabriel (bberry mail)
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