The first thing to do would be to try to install the latest version of Lightroom. If you are using LR5 then it would be LR 5.7.1.
From Googling, it appears to fix, at least when the error happens during publishing photos to Facebook, is to delete a couple cryptic lines out of the LR Preferences file. To me these lines look like an encrypted password.
Since you're having an issue with e-mailing, maybe you'd need to find the corresponding lines for your e-mail rather than Facebook, unless preferences stored for FB are also being interpreted when you sent an e-mail.
The LR preferences file is a text file so you can use a text editor like Notepad to make changes to it. It is a good idea to make a backup copy of it, first, in case the editing doesn't go as planned.