Have you checked to determine if this is also occurring in the actual mail service which is the web mail service? Your Mac mail is a replication of the web service. If it's also there, you need to contact them.
My first thought is somebody is carpet bombing your webmail account and they're being downloaded into Mac mail. I guess that could also occur if multiple folders were added to your webmail account, but that's just a guess
Either your OS is causing this or an outside source is.
I suggest you change all your passwords in an attempt to stop the possible incursion. It might be a good idea to drop the use of your current web mail account and create a new one with a different password; at the very least change the password. Yes, this is a lot of work and hassle but what choice do you have?
If it were me I'd create a new Admin user and remove the old account retaining the old one's Home folder and data. Your user profile may be corrupt in some way.
Th idea is to cut off who or what is causing this and many times the actions required are several and tedious. Also, the likelihood of taking a single action and having total success is slight. That's why several actions are suggested.
Have you considered malware? If not, download MalwareBytes and run it: https://www.malwarebytes.com/
It's safe; you don't need the paid version for a simple scan.
