80+ Mail console messages per seconds: "no entry for mailbox id  in the cache; this should never happen"
Same problem for me and this have solved :

Quit Apple Mail

Go to folder : ~/Library/Mail/V7/MailData (you should use the Finder &quot;Go&quot; menu then &quot;Go to folder&quot; because this folder is normally hidden.
~ is your user home folder.

Rename all &quot;Envelope Index xxx&quot; on &quot;--Envelope Index xxx&quot;. You should normally have 3 (on the screenshot I have 5 files but it's because Apple Mail was launched when I take it).

Launch Apple Mail


Normally, Apple Mail will re-create this files that are cached data and all could be solved.
Jérôme.