Are you using docker compose? You can set the container hostname for telegraf, https://community.influxdata.com/t/incorrect-hostname-picked-up-by-telegraf-docker-container/24469/6

Not directly to your exact question, but do you have something like rspamd set up? I would consider doing some filtering with rspamd modules. It’s a lot of learning to go through, but has some good defaults. It can filter outgoing mail as well as incoming.

Should be able to handle a few users OK. You might want a more permanent server to run it on if they want to keep using it long term though.