"System Administrator" removing emails from server before I have seen them

[#1]

Now and then, I receive an email from someone (people I know) but instead of their content, the email is headed System Administrator message unavailable.  And their message is "This message is no longer available on the server".  For one which happened early yesterday I looked on Plusnet's webmail and indeed there was nothing there for yesterday's date.

I had wondered if somehow my "storage" was full but I don't see any setting which relates to this.

I use a current  iMac and this came (as usual) with Apple's Mail app.

Any ideas on what's going wrong and how to fix it?  Plusnet's phone help were baffled by the intermittency of the fault.

[#2]

@Drayton wrote:

I had wondered if somehow my "storage" was full but I don't see any setting which relates to this.

Does this work ?

[#3]

Thank you for this guidance.  I found my way to the IMAP Mailbox by control-click on "All Inboxes".  In 'Account Info' there is a horizontal quota bar  0% -  50% - 100% but the bar is grey and not working. There is no statement of the quota limit. Fortunately, beneath it is a list of mailboxes, archive, bin, etc, with a storage sum for each.

I have learned that it is up to me to keep deleting items -  and erasing the bin too -  to avoid unseen emails being destroyed.

I have also learned that a powerful iMac with huge complexity I don't need or want, is something of a burden.