Mail oddity 1

Here in the UK, one of our major retailers has been supplying a broadband and phone service for several years and has many satisfied users . However, due to some business decision they are closing down the entire operation during the coming summer. The service was actually provided by Plusnet (which in itself appears to be part of BT, and had very good UK based customer support).
The retailer is advising all their customers to switch over to another service - Plusnet as it happens - so the support will still be the same.
One of my friends is being switched next week and has asked me to help him make sure it all goes well.
He does though have couple of issues that are puzzling.
When he launches Mail he gets a box saying something to the effect of “Mail can’t do two things at once” then after a short delay Mail starts up as normal.
The other may be related, when he deletes a message or tries to move it to a folder in mail he gets an error message to the effect that the move/delete can’t be done - then moments later it happens!
As far as I’m aware he’s up to date with his OS .
Any thoughts about the issues above?

Sounds like something up with the mail database, rather than the ISP he is using. He might benefit from rebuilding his mailboxes.

But make sure he has a backup, such as Time Machine, before rebuilding the Mail database. Maybe also make a copy of the Mail data on the Mac (~/Library/Mail ? ).
As I understand it, Rebuild downloads all (most?) of the emails from the ISP. If the ISP is having issues then emails might be lost. The Apple link suggests it is just an indexing function but in my experience the Rebuild takes a long time because it is downloading emails.