MY ANSWER:
I have had this too. Office 2010 and the email client is THUNDERBIRD.
When I have occasionally had this problem, simply closing thunderbird completely and retrying the save and Send PDF option works ok. For me that means the issue was with the email client (thunderbird) having a hissy and closing it (for it to subsequently restart cleanly) fixed.
Reboot wasnt necessary and I never thought for one minute that its anything to do with AV software. (To suggest so is very wrong. I mean how come you can save the sheet NORMALLY and yet not with the 'save and send' option?! In both cases its the same system with the same security software running)