Yes check the Deleted folder on the left of the main folders. The after emptying deleted items then sometimes a reboot is necessary to see the space freed up.
I think that you cannot easily see the space occupied by email but it is important to empty mail Trash too as some attachments might mount up in size. I seem to remembrr it being said that on the free account the email space does have a limit so “Mail full to limit” might trigger the warning even if the “main” storage occupies only 184MB.
If you happen to have saved messages with (large) attachments maybe it is a good idea the Save the attachments to some other folder out of email.