Process "mail networking" stalls Apple Mail

I hope someone can help me, as I am getting desperate. One of my e-mail accounts at work is an Exchange account (which may or may not have to do with the issue). At some point almost a year ago the text window in Apple Mail started to remain white when I selected an incoming e-mail on that account, not showing the contents of the e-mail. Sometimes it showed up after a minute or so, at other times it eventually reached the limits of my patience.

I finally discovered that the problem was connected to a process called "mail networking". If I kill it, the e-mail will immediately be displayed. However, switching to the next e-mail results in a repetition of the experience, and having to kill mail networking with every incoming e-mail soon got old.

I tried everything I could think of, including deleting the database, all e-mail accounts and the associated folders and mail preferences, no joy. I eventually got a new MacBook Air, on which everything works just fine. I took the old iMac home with me for remote work, wiped the harddisk, installed a fresh copy of OS X, created a new user, and set up Mail again. After that, e-mail worked fine for about 2–3 months, but now the issue has reappeared. It's driving me crazy. The machine is running Monterey (the newest OS X available for it), but when the problem first showed up it was running Big Sur. I also have an even older iMac at home on which the problem does not occur.

Any help would be greatly appreciated.