At some recent point, Mail has begun deleting all my messages more than a month old. I have looked for a setting in preferences which will allow me to extend the length of time messages are saved for, but such a thing no longer seems to exist! Can anyone help?
So, that only happens to messages that you put in Mail's trash?
That's usually because you don't need to keep them, correct?
Deleted messages go in the trash.
Sounds like your trash is just too automatic for your purposes.
If you want to be sure about keeping your messages, then don't put them in the trash (Don't delete your messages, if you really want to save them… )
Make a folder in your account - maybe more than one if you have multiple interests, and drag your messages to the folder of your choice.
That extra folder will keep your messages fully under your control.
There is a preference setting for how long you want those trashed messages to sit in your trash can.
Go to Mail Preferences>Accounts>Mailbox behaviors. This preference must be set for each account you have listed in Mail.
DeltaMac has a point. If you didn’t want those messages to be deleted, why did you put them in the trash?