1. https://serverfault.com/questions/117906/what-happens-when-a-mail-server-goes-down

“The SMTP protocol is very resilient in design. Most modern MTAs will attempt to resend a message every 4 hours for up to 5 days though this is configurable by the particular systems admin there is usually very little need to change from this.”

2. Get a clean IP, there are many online sites that offer to check an IP: http://www.mxtoolbox.com/

Then make sure your server is configured well so it doesn’t get a bad rep: https://www.linuxbabe.com/mail-server/setup-basic-postfix-mail-sever-ubuntu by setting up SPF, DKIM and DMARC.

You could do what I did and move the mail server to a local machine or VM then use Wireguard on the VPS which tells iptables to route incoming mail traffic ports to the now local mail server. Then you can use as much space as you want.

Dewey decimal system IIRC from my school librarian days in the 70’s. I’d search like this:

https://github.com/topics/dewey-decimal

This might be a start:

https://github.com/SardineDude1

I’ve self-hosted for over two decades and most of the apps you mentioned I’ve not heard of or used. I self host email and my web apps and system services I wrote. I do it because I like it.