Right now I have, Maximum allowed file size is 16 GB.
That’s a lot.
I had not even noticed that and mine is the same way. I think a large number of mail servers out there will not even support receiving email with attachments above 10MB - 20MB in size.
I don’t think we actually support that…
Don’t support what, a way to change it, or somehow magically mailing a 16GB file attachment? 
Magic mailing. I don’t think it will accept it. Sane limits are set for Postfix and Exchange in the setup scripts.