According to their website, Fastmail allows up to 100 custom domains on a single account, so it's not like you need to buy a second account if you just want to receive/send email on a domain.
Right, but I don't really see why you would need separate accounts. In my experience a single account for multiple projects is actually very convenient (with rules to automatically sort incoming mail).
In my opinion, multiple accounts are really only necessary if you have employees or collaborators and want to make sure that not everyone can access your stuff.
But that's just my point of view, I understand that what works for me may not be suitable for your projects.
The way that users work with Fastmail, is that each user has a single email address, the rest just acts as aliases to that address. So when receiving works, sending always happens with the main address.
For small projects, I don’t necessarily want to reply with my main address, hence why I want to have it seperated.
