Some good info here:
https://help.directadmin.com/item.php?id=514

There are 2 global limits:
1) Per-DA-User limit:
/etc/virtual/limit

2) Per-email account limit:
/etc/virtual/user_limit

Both can be set at:
Admin Level -> Admin Settings

The value you'd be interested in would be
Max limit User can set per E-Mail = -1

Where -1 means the value a User can set in their E-Mail limit would be the global limit in /etc/virtual/user_limit.
That's likely set to 200 right now.

What you want would either:
Max limit User can set per E-Mail = 0

or:
Max limit User can set per E-Mail = 1000

so that new email accounts still default to 200 (if blank or unset), but allows the User to go up to 1000 is needed (sends can't go above DA User limit though).

John

https://forum.directadmin.com/threads/email-account-send-limit-errors.53696/