As stated at https://forum.emclient.com/t/from-postbox-to-em-client-worth-it/101685/50?u=postbox

Probably the biggest failing in eM Client is the inability to set synch periods per account, to lessen the impact on mail servers. With 150+ accounts, and no way to control synch, even a dedicated mail server will struggle, never mind shared hosting.

I agree that this would be very helpful even with just half a dozen accounts.