My first thought is that you have Gmail set to import from your sending account. That could explain why messages you send end up there. I do not understand what you mean by having your mail on the same Thunderbird page.

Thank you David.

I have reduced my Thunderbird page to just two addresses i.e. my private account and my Gmail account, I did at one time have four different accounts on the Thunderbird page so I could view and answer emails as received without having to use separate email software, this has worked 100% until now. I have not done ANY alterations to my Thunderbird account data this has in the past few weeks acting like this however I have now removed my Gmail account from Thunderbird to see if my private account remains normal.

Thank you for your input.

Select the lyn.co.uk account in the left pane of Tools/Account Settings, then look at Outgoing Server (SMTP) in the right pane? Is the gmail smtp selected? If it is, that would explain why your recipients see messages as if they are sent from the gmail account, and so when they reply, it goes to that account. Gmail will automatically do this unless the lyn.co.uk account is entered as a 'Send mail as' account in gmail settings, Accounts & Import.

The better alternative is to point Outgoing Server (SMTP) to the smtp for lyn.co.uk in the drop-down. If the server hasn't already been created, do so in Outgoing Server (SMTP) at the bottom of the left pane of Account Settings.

https://support.mozilla.org/en-US/questions/1324013

Thank you again.

Yes I agree with your 'train of thought' but rest assured the the data fields are correct and 'doubly' checked.

As the email address is a business address I can do without problems therefore I have deleted Thunderbird and now using a paid software which is designed for multi addresses so we'll see at least I can shout if their software also causes the same issue.

I am extremely grateful for your thoughts.