Please elaborate a bit more clearly explaining exactly the process you are using and state where you're 'additions' are inserted.

For example: select email so it displays in Message Pane. Q: Do you select the text before clicking on 'Reply' or not ? click on 'Reply' button

Q: Where are you typing? Above or below or within the sections of the previous email like editing sections?

Please post image showing the following window: Right click on mail account name in Folder Pane and select 'Settings' select 'Composition & Addressing' see image I supplied below - post image showing what you have.

Hello. Thanks for replying. I usually just right click on the incoming email and choose 'reply to sender only'. Then I write my message underneath what I received. Sometimes I delete some or all of the content before replying. I hope this helps. Just to complicate things, some of my replies do get through. Best wishes Bernard

Do the emails you send get saved correctly in the 'Drafts' folder whilst you composing the email and also in the 'Sent' folder after sending ?

I don't know about the drafts folder, but the mails do appear in my sent folder. I have tried to reproduce the problem, but the tests worked as they should.