Can this scale to 10's of 1000's of users on a big server? I wanted to open an alternative to gmail where users can use their domains and open several mailboxes for a small fee but when searching for solutions I always end up with dovcot scaling which is hard or expansive. What I am searching for is some solution where the mailbox can store large attachments on S3 / b2 and maybe also email content and just index the inbox on a local DB / Redis. Is there something like this available open source?