I have done as you say but so far nothing has happened. How long should I wait to see any result?
I typed in what you suggest but nothing comes up. I have looked in Library file and there is no Mail file or folder.
Plusnet is my ISP but I do not use their email, I use gmail. I have recently cleaned out the gmail account so some of the old emails are not there.
If you went into Gmail.com and deleted some emails, it seems very likely that by doing so they also got cleared out from your Mail app.
It isn't that the emails have been wiped out, they still appear in the file when it is opened from the left hand Mailboxes column. When opened from the right hand pane there is nothing in the content.
Not really because older emails are still intact and there is still a file entry in the left hand window.