I do this too (3 domains, Fastmail) For specific vendors where I am at the shop, I just make up an alias email with their name in it. For apps, services, I co

I do this too (3 domains, Fastmail) For specific vendors where I am at the shop, I just make up an alias email with their name in it. For apps, services, I configured bitwarden to create email aliases on Fastmail, so they are linked to a service.