To your multiple IMAP concept, I have been using isync / mbsync (name change, package isync in Debian) for years running via cron script to pull email from one domain at one provider and push it to a subfolder of another domain at another provider. You have to be aware of one specific gotcha but it’s otherwise been working all by itself forever without issues. Take note of the PipeLineDepth 1 for IMAP service providers which throttle your speed, I have to use it on the destination side provider config.

Two tips having worked in the corporate world (strict controls):

• Create a basic non-spam web page for it that has something that doesn’t look like SEO garbage or whatever. Nothing more than “hey this is a personal domain of the flatbield family” is fine, maybe a link to something (links enhance rep - put a picture of your dog up or link to a wikipedia article or something) and let it rest for at least 30 days. The 3rd party filtering services used by corporate players severely limit, block or distrust a domain newer than 30 days (or longer, depending). Set up a SSL cert on it for another +1 to it’s rep value, HTTPS is looked at by these services and ensure the CA record is in your DNS for that SSL issuer.

• Ensure you use the Providers’ setup for DKIM, SPF and so forth (many like Fastmail have a DNS-check wizard to get you all set up) as many modern providers will instantly downvote you if anything is missing or wrong with these controls (I’ve heard GMail and O365 particularly). In 2024 these are a must-have, not a nice-to-have, for getting your email received by anyone and everyone.

If you chose a domain at a TLD which has/had been used by the bad buys (dot-xyz, info, zip, etc.) you may wish to reconsider - there are TLDs which are wholescale blocked or downvoted in rep based on this (by the same services used above). Ensure someone working at a bank (strict egress controls for their employees) can visit your domain as a good litmus test as to it’s validity for use in email reputation.

A company such as Fastmail spends a lot of time ensuring their IP address space for sending and receiving mail is clean - getting spammers off their service, getting IP rep cleaned off blacklists and so forth. So your task is to focus on the same thing for your domain - if someone had previously owned the name they could have gotten it on blacklists long ago, a handy way to check old history is looking it up at web.archive.org for captured snapshots (and I’ve walked away from domain names because of this once I discovered previous content I didn’t like).

Fastmail has one feature many others lack (which is hard to research unless you want/need it and have go down the rabbit hole) - scope limited login tokens for specific uses. Specifically, you can set up one for “read only IMAP” (to archive emails using scripts etc.), “SMTP only” (to send emails from scripts like backup reports etc.) and so forth. Many, if not most, other providers either don’t have it, or if they do it’s very limited like one token only with no scope control. $0.02 hth