You don't need a mail server to send mail. One can send mails through a VPN as client.

With Sonic I have to use their servers for outbound stuff since they block outbound SMTP without a static IP (and they don't offer static IPs with fiber). It's a price I'm willing to pay since I typically don't see false positives (and ye I check my logs periodically) with Spamhaus.

Unfortunately I've moved to Proton and the increase in spam is pretty damn frustrating.

Spamhaus is blocking by IP which can be an smtp server or a client. The SMTP protocol does not allow to distinguish a sending SMTP server from a client.

By using a VPN, you "randomize" the IP address and thus make spamhaus and equivalent services useless. I created my own IP blacklist and tracked it.

The only method I found to filter my spammers is to reject mails from hosts without a name. This eliminated 80% of spam, but it won't last long.

The mail system is completely broken.