luck was what I needed. I had tried unsuccessfully to get it working for maybe one hour, before posting my question. I tried again, and this time (after another hour) IT WORKS.
The trouble was not the regex search pattern. Not because I'm such a genius, but because I could google for it. The problem that took me so long was getting the macro to take the regex result of search in the email for the domain.
I did not understand how to do that, but by first trying the appealing versions and then the weird versions (from my inexperienced point of view) I stumbled over the right one that works. I always thought that the "%REGEXPMATCH=%TOADDR" bit would get the domain string and I would use the %REGEXPMATCH then again in the IF part, you know like a system variable. Like so:%IF:"%REGEXPMATCH"="@example.com": ^^^^^^^^^^^^^^ thought this would BE the regex result, and IF it's the correct domain it would be like "@example.com" = "@example.com" -> TRUE
Didn't work. Only when I used that full code piece in the IF part, it worked.
the working part results (I think) from the macro's POV in:
BECOMES with the wanted domain:
%IF:"@example.com"="@example.com": --> TRUE
%IF:"@wrongdomain.fr"="@example.com": --> FALSE
Learned something about TB macros, and I hope I will remember it at the next time I'll try do something nifty.
So this I just put at the beginning of the template, this is the regex part 1 - setting the pattern
to see what regex actually finds (not needed after one knows that part works), it outputs what the regex sees as the domain part in the TO field:
then here comes the thing that does the magic:
%IF:"%REGEXPMATCH=%TOADDR"="@example.com":%SUBJECT="insertyourtext - %SUBJECT"%-
The %SUBJECT="insertyourtext - %SUBJECT"%- prevents the deletion of whatever text one was typing when going through the header fields in a new mail.
So one has to type into that macro the "example.com" domain and the text (like in my case my customer ID) at "insertyourtext".
all this I have put into my macro that gets started when I open a new message window.
sorry for the very long entry for a very short solution.