Mail settings for 100+ mailboxes

[EndErr]

Having just 1 client with 1 domain and a lot (100+) of mailboxes there are problems with mailserver connections, in special with imap.
I use Thunderbird for mai client.
I did change settings for "Max SMTP Instances" from 200 to 2000 and "Send Limit (per-account per-day)" from 300 to 3000. Not activated:r filters / antivirus / spam checking or firewall ...
When user checks the mailbox some times a error connecting to imap.mydomain.tld appears.
Another error is when the message is saved into the Send folder.
But the most time the mailbox is not accesbile and messages could not be sent.

Are there any settings for such configuration?