Hi thank you for your response. I use IMAP but will try and change to POP to see if it solves the issue.
I only use Apple Mail and delete all my emails there. I had no problems with the rest of the functions until I received a warning from Swisscom that my mail was full. I then went to my bluewin.ch site and discovered none of these emails had been deleted from there creating double the work.
before that I had a pc with Microsoft. and never experienced any problems there. I believe that was a POP account. I will check these two types of accounts out to find out the pros and cons before making the switch as I do not wish to risk losing any data.
as Swisscom shop sell Apple products I thought would be compatible with Mail. I had bluewin.ch long before I used Apple devices. If this is not ge case I will have to resort to iCloud. The bluewin.ch site does not display well on my iPad Pro and wastes time whereas Mail is always available and for convenience I prefer to have everything in one place.