Now this account in Mail does not work at all, and I get this message every time I try to take the account online:
"Some actions taken while the account “laurenmurch” was offline could not be completed online. Mail has undone actions on some messages so that you can redo the actions while online. Mail has saved other messages in mailbox “On My Mac” in “On My Mac” so that you can complete the actions while online.
The additional information part of the message is not always the same. It has also said "Additional information: Operation could not be completed. (MFMessageErrorDomain error 1028.)" among other things.
(Update: I REMOVED the account and added it again, and the problem persists.)
(Update: After trying several other things, I deleted the account a second time and restarted mail, and seem to have solved the problem)