

I understand this, but this is inconsistent behavior. You now use 22 inside your network and something else outside. Whenever you create inconsistent behavior, everyone using it has to have an awareness of all these inconsistent behaviors.
Also, it is hard to troubleshoot because the tool most admins would want to use (netstat) will not give you useful information to understand the situation.
I would recommend something like stalwart, which is just a single binary and works. Gives you a web interface and a zonefile you can just copy paste into your DNS including all correct DMARC DKIM SPF and autodiscovery records.
Setting postfix, dovecot etc. up from scratch can be a bit time consuming and annoying.
Deliverability depends on where it is hosted, many VPC providers IP space is completely blocked in spam filters.