I just looked at the exporter code only handles PMs you received, so no Sent PMs are/can be exported. I’ll look at the code to see if it’s possible to create a Sent Exporter version but it’s not a simple mod. I think I’d need to duplicate the entire exporter and then make some significant changes to do it.
No, don’t worry!
If you don’t try, you don’t fail…or succeed.
I’ve created a custom modification that has two options…one to export ALL your received PMs and one to export all your SENT PMs. The SENT option is on the same menu as the ALL option.
It seems to work but I’m not 100% sure whether all recipients (non-bcc’d anyway) are properly listed. As sending PMs to multiple people isn’t very common I’m not going to worry too much about it. Labels for sent messages aren’t displayed either, but I don’t think they’ve been extensively used and I’m not really sure whether they apply to sent messages.
Please let me know if this works OK.
Fantastic 
1.4 MB this time. . .
Very many thanks 
Chris I think I’ve found my issue with Firefox and it’s nothing to do with discourse but what happens when it updates from earlier versions where addons are not supported and they removed access to about:config which means I am unable to change internals. Extremely annoying but unlikely they will return access. A real PITA.
Stuart