Mailbox Full error, very little email in mailbox

I tried sending an email earlier that did not leave the outbox. I thought it was an error on the receiving end, but looking at the error log it shows ‘mailbox full’. Meaning MY eM Client email mailbox. I download my mail and the majority of it is in the local folders. (My server shows I am using 10m of space to store email.)

I deleted all the trash. No good.

Then I deleted one email from the inbox (where I have 6 emails sitting) and then the email in the outbox was sent.

Obviously, I’ve hit some sort of wall somewhere, but I can’t see where. If deleting one email out of six, when I can have over 30 emails sitting on any given day, allows the outbox to function well that seems odd.

Something that may be relative to this problem - I got a new computer earlier this year and had to transfer all my email from the old computer to the new one. The old emails sorted themselves out correctly (with some help from you) into the appropriate folders (local and server). However, there is now, and has been since the transfer, a folder called ‘Unread’ in the Smart Folders that contains every email from the transfer and every unread email since then. Is this where I am hitting the wall of mailbox full?

If so, how do I get rid of the emails in the Unread folder in the Smart Folders? I cannot delete them as I tried one and it deleted the same unread email in the Local Folders.