Postfix - Virtualmin - Message size exceeds fixed limit 554 5.0.0 Service unavailable - Max email size ?
That error sounds like users are trying to send your server emails that are larger than it's configured to accept.
You can see what the current limit is by running this command:
postconf | grep message_size_limit
If you'd like to increase the limit, you can go into Webmin -> Servers -> Postfix -> General Resource Control,
and set "Max size of a message" to your desired message size (in bytes).
So, to set it to 25MB, you'd use the parameter "25000000".
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