This can also happen if the web mail is corrupted and some part of the web mail is not working so you can try out the following steps ...

Go to the outbox directory in Windows Explorer. The location will be:
%USERPROFILE%\AppData\Local\Microsoft\Windows Mail\Local Folders\Outbox
and with WinMail closed delete all the files.
Then try repairing the database.
Run the cmd prompt as Admin and copy and then in there
esentutl /p "%USERPROFILE%\AppData\Local\Microsoft\Windows
Mail\windowsmail.msmessagestore"
Fill in the %USERPROFILE% to match your profile (e.g. c:\users\nrc)
Then reopen WinMail and see if that fixes it.