https://bugzilla.mozilla.org/show_bug.cgi?id=1356902

Open Settings/Apps/Default apps, click on 'Set defaults by app', select Thunderbird in the left pane, click 'Choose defaults for this program', then check all the items, including MAPI, Save. Does it work now?

Many thanks for your answer..

I have Thunderbird running on a Windows 2003 Server and the options you describe aren't shown I'm afraid..

Do you have any further suggestions please, bearing in mind there have been no changes in the server software and everything was OK until the last update of Firebird.

The instructions were given for W10 since that is what's indicated in the details of your original post. But there must be an equivalent way in Server to specify default programs, in case the original settings have been lost. The closest I can find is this article that explains how to do it in Control Panel (use 'Custom' for Thunderbird).

Hi,

I'm sorry but this isn't the answer as I don't want Thunderbird to open when I click a file, I want it to open a new message when I click 'send it to'

I should have mentioned that a lot of people are having the same problem with this issue on various Windows platforms and a bug in Thunderbird has been reported.

If it isn't working for you on Server, as it does here with W7 and W10, you may want to downgrade to TB 45 until the MAPI issues are fixed in the current release.

There is a workaround in that instead of selecting 'send to' you can select 'open with' then chosing Thunderbird.

Busby said

There is a workaround in that instead of selecting 'send to' you can select 'open with' then chosing Thunderbird.

Nice, my friend. Works like a charm! Thanks.