I want to host mail storage myself, mainly.

Posteo, for example, is extremely expensive there. At 0.25 EUR per GB per month that would easily be a three-figure charge per month.

I do have the infrastructure for backing up the data on the server, but archiving email outside of IMAP and managing my data not exceeding 20 GB or something is not what I want. My mail accounts can be hundreds of gigs and that is fine.

My question is mostly about SMTP, which I do not want to do myself.