Your own mail server on your own domain. A Raspberry Pi in a data centre has a fixed public IP address, which a mail server at home almost always lacks.
What runs on it
Postfix for sending and receiving, Dovecot for IMAP, Rspamd against spam and Roundcube as webmail. Set up SPF, DKIM and DMARC records, otherwise your mail will end up in spam at the big providers. Ask support about a reverse DNS record for your IP address.
sudo apt install postfix dovecot-imapd rspamdWhen hosting is the better choice
A mail server means extra work: updates, keeping an eye on blacklists, backups. If you just want email on your domain, ordinary email hosting is simpler.