Setting up mac mail to use correct SMTP server for different email addresses

How do I get Mac Mail to use the correct SMTP server for each email address?

I have a vanity domain.

When it receives email (it has multiple aliases) it forwards them all to a Gmail inbox.

I need to send email through the domain's SMTP server to get the headers right.

Apple Mail lets me add multiple sending email addresses to one account.

But each account only has 1 SMTP server, so I can't say,

"For this address, use this SMTP; for that, use that".

1st Question: Or can I?

Another way is to set up 2 accounts: My Gmail Account, and the Domain Account.

Apple mail lets me set this up, and configure the correct SMTP servers for each.

BUT the domain account HAS NO INBOX, so I get a heap of unwanted warnings about being offline.

2nd Question: How do I set up an account without an inbox?

New feature Suggestion: When setting up an account and choosing between IMAP and pop, please include a "none" option that gracefully turns off inbound email checking...

PS: This is using High Sierra on a 2009 iMac. Excellent hardware, Macs last ages....