Sounds a bit less good. If you only have one server now, then
partitioning out by function will probably be *much* easier.
Everything except dovecot is very easy to spread out across
servers, so to take the easy route I would probably keep dovecot
where it is, and spread out the other functions on new servers.
You can have one or two sendmail servers, two or more AV servers,
and add to those as needed. I'll bet that your "not enough"
problems are more related to the AV functions than to dovecot.
It's not completely true.
The trend in recent years has seen the increase in the size of the
mailboxes as well as their number.
It's true what you say, so you partially won, but consider that virtual
server provider I will use offers machines with a fixed mix of resources
(instance A with 4vCPU/8GB RAM /100GB SDD, B with 6vCPU/16GB RAM /200GB
SDD etc), no block storage available,
and that I'm actually hitting low disk space with 1.5TB, I will waste
vCPU for storage/dovecot instances.
P.S. BTW... Sendmail?
I'm using sendmail but I thought that in an environment like the one
proposed was better postfix with virtual users, any suggestions?