This is not very easy to follow. At first I though you were creating a new mail and had set up a 'Reply-to' 'reply to' in dutch is 'antwood naar'

But now I see it is not you creating a new email. You are replying to an email.

That means the antwood naar auto inserts whatever your account has set up use as the Reply-to' email address which in this instance is the 'activities' at bridge club email address. I can see there is an email address entered into the 'antwood naar' field.

There is nothing in the Aan (TO) field - were you expecting something here ?

Your response is at the top Your signature is the text which displays above the reply section This is because you are using the Account Settings > Composition & Addressing for your email address To auto quote the original message - start my reply above the quote and place my signature below my reply above the quote.

The original email which you are replying to is copied below It says on a date wrote the secretary of bridge club which says to all members of the club.... That original message also displays a signature which is RBC and an image. That signature would have been included in the original email to which to are replying.

But the FROM also says it is the secretary of bridge club.

It looks odd because it almost looks like you have sent this email before.

You would need to explain more in order to make more sense because at this point it looks like to are replying to your own email ?

Many thanks for your extended reply. Sorry for my unclear description. I have done some more digging into this issue.

First I created the mass mailing on Thunderbird on my main computer with the account "secretaris@rijensebridgeclub.nl". Then on the same computer with the same Thunderbird client I replayed from my personal account. That gives the screenshot that I included.

If I replay from from an other computer or client it all works as expected.

Then I went back to my "secretaris@rijensebridgeclub.nl" account section "send items" and replayed on the send email which gives the same screen as the included screenshot. Very strange. It looks like Thunderbird mixes data from different accounts on the same client.

I hope this clarifies the issue.

I don't completely understand this, but the problem of accounts getting mixed up with sent and replied messages happens if the accounts are sending through a common smtp, instead of each account sending on separate smtp servers. Even if the accounts are on the same domain (part after the @). Check your setup to see if this is possibly the problem:

https://support.mozilla.org/en-US/questions/1350593

Thanks for your suggestion. Indeed I mixed up a common server over three over three different accounts. It is the provider of these accounts is the same. So for each account I created a separate smtp server with of course the same smtp server. Also I was not very careful and used function names and my own name not very structured. So now there are three different separate persons linked to each account.

Now I tried with all received messages send by me over the last year with a replay to email addresses and they all behave the same. On my Android tablet with TypeApp it works fine. I live with it as I know now that this does not work correctly on the same machine but I can use the feature as on the other end it works as expected.

Many thanks for your suggestions and help!