E.g. I'd type John Smith and then select the right one from the drop down list and it'd appear as:
"John smith <js1234"@yahoo.co.uk>
Which is of course rubbish.
Looking in the address book for this entry I had:
Email: john smith <firstname.lastname@example.org>
Tried playing with the address by cutting down to just the address and other stuff - no difference, I still got the problem.
Eventually found that by adding a display name it sorted it all out. So when it worked I had changed it as follows:
Display: John Smith
Simple really but I couldn't find anything by quickly searching the web (other than this forum link which didn't really help me) so I've put it in here.