what to do when mailbox is over quota

You can follow the article : Mail for Mac: Import or export mailboxes

Once the messages are in a new mailbox called Import in the Mail sidebar. You can drag folders and messages from the Import mailbox to new or existing mailboxes, then delete the Import mailbox.

Delete a bunch of old messages.

saz172 wrote:

Ive done that but its made no difference. Any other ideas ??

Empty your deleted items folder.