dkim customizations (bug: migration invalidates dkim signatures on previous emails)

[locknut]

I'm migrating email for an existing domain to simplelogin.io, this domain has existed for 10+ years and has already had DKIM setup with the selector "dkim".

Prerequisites

• I have searched open and closed issues to make sure that the bug has not yet been reported.

Bug report

Describe the bug

To migrate to simplelogin, I'm obliged to replace the contents of that specific existing DKIM DNS record with simplelogin.io's supplied DKIM DNS record value for the same selector ("dkim") that's been in use for 10+ years. This is fine for all future emails, but unfortunately many email clients actively re-validate DKIM signatures every time an email is opened, and will present a rather scary warning to the user if DKIM validation suddenly starts failing for old messages - even if the message headers clearly indicate that DKIM passed at time of receipt.

That's a problem I'd really rather avoid.

Expected behavior

Options:

1. The ability to import custom public/private keys for DKIM (while continuing to use the existing DKIM selector names - dkim, dkim02, dkim03), on a per domain basis.

2. The ability to modify the DKIM selector values (while still using simplelogin.io's signing keys & DNS records), on a per domain basis. So instead of this:

dkim._domainkey.mycustomdomain.com.    IN   CNAME     dkim._domainkey.simplelogin.co.
dkim02._domainkey.mycustomdomain.com.    IN   CNAME     dkim02._domainkey.simplelogin.co.
dkim03._domainkey.mycustomdomain.com.    IN   CNAME     dkim03._domainkey.simplelogin.co.

I could do this:

dkimsl._domainkey.mycustomdomain.com.    IN   CNAME     dkim._domainkey.simplelogin.co.
dkimsl02._domainkey.mycustomdomain.com.    IN   CNAME     dkim02._domainkey.simplelogin.co.
dkimsl03._domainkey.mycustomdomain.com.    IN   CNAME     dkim03._domainkey.simplelogin.co.

3. The ability to disable specific DKIM selectors from being used, while keeping others enabled, on a per domain basis. If I could just disable "dkim" while keeping "dkim02" and "dkim03" enabled, then that would solve my problem.

Screenshots

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Environment (If applicable):

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Additional context

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