I would do this with LVM. You can grow LVM to expand onto a second volume and expand the file system with it. Or ZFS.

https://wiki.archlinux.org/title/LVM#Add_physical_volume_to_...

However note self hosting mail used to be super easy but these days most report you have a real hard time getting people to accept your mail. When your IP has no prior history and small volume you’ll get false flagged as spam a lot. Likely to be your larger challenge.