How to unblock "mail outgoing" for good ?

How to unblock "mail outgoing" for good ?

Approximately a week ago I first got the message:

User : jenny at saulwickartonline dot com
Details :Mail Outgoing blocked. Reason :Daily Mail Size Quota Exceeds Limit [Total Mail Size=512,000]


I checked the email I had sent in the preceding 24 hours, the size was really small (a few MB).
The mail storage quota was given as 5% used.

I did not understand the message about the total mail size (512,000). Is the unit MB? Is this total mail size that of all all mail sent in the past? or of the mail sent in the last 24hours? or of the current mail storage?
My mail storage was shown elsewhere on the interface as being 5% used.
I am using an IMAP client and my total storage remains very low.

There was a link to unblock outgoing mail on the Zoho mail interface. I used it and outgoing mail was unblocked.

Following this, once or twice a day I receive the same message after having sent a few messages.
When it happens I unblock the outgoing mail again following the link, and the cycle starts again.

I have since reduced my total mail storage to 205MB, but to no avail: mail outgoing gets blocked again and again and the same message is received.

My settings may be wrong and some mail could be accumulating somewhere. However I do not know where some quota may be exceeded so I cannot fix this issue.

Can someone please tell me what is happening and what I should do to get mail outgoing unblocked for good?