So back in the day 2010 we all got a sweet deal from Google to host all of our emails with any number of domains for free. I host multiple emails for my domain like tech, buy and billybob @xyx,com plus a few other domains. seven in total. So as I use separate inboxs for all of them that’ll be 6 X7 $42 per month - bugger that!
So my solution is:
MacSparky approved Fast mail for billyBob@xyz.com - counts as one user $5 pm and then perhaps tool around with the new iCloud + custom domain options free for me 2TB sub. and my other extended family too who also are subs. $1.99pm?
Also my host offers Titan mail for $2.50 a month but not heard of this.
Very happy Fastmail customer here. You can create as many usernames within your own domain as you want without any additional charge. The additional charges would be per person on your account, not usernames on the domain.