Probably mailbox settings - go into options and look at the various fields that can be changed then Google ‘custom domain email settings for xyz’ - usually its server settings and you just edit the text till it works
Try setting your custom domain up as an IMAP account that's separate from the @mac/iCloud account, then have the @mac account forward to the custom domain. I know this is super un-intuitive by Mac standards, but it works. Here's how I have mine (Zoho instead of @mac, but it will work the same):
thx! I have a work around after an hour w Apple Support, where he somehow found a way for me to both send from the custom and set up a smart mailbox that reasonably catches what it needs to as an inbox
I'll probably end up getting a different *actual* account from squarespace or something later
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ie my @ mac account gets everything from the custom, but I don't have the option to send FROM the custom
any thoughts, hive mind? MacOS 15.3
if I "enable" the custom server it nukes my mac account
I'll probably end up getting a different *actual* account from squarespace or something later