Messages deleted after being read

Apple mail/g.mail suddenly deleted several years' worth of saved correspondence from a friend (I opened the mailbox, and the mail was gone).

Now all messages vanish upon being read.

They are not in the trash.

They are not in spam.

They are sometimes still online.

I made no changes to my mail settings; this all started happening spontaneously.

I installed the Sequoia system in hopes it would help. It didn't.

In desperation I installed Spark, which keeps the messages, but does not receive any from my friend. I'd rather rely on gmail, which has worked for so many years. Until now.