@sadmicrowave have you checked for a folder /home/[myuser]/.evolution/mail/local/inbox?
Might be stored under that directory (if not erased) as that is where 11.04 stored some of my old mail files and always backed up those copies prior to upgrade for re-import post-upgrade (just as a precaution and regular step of safe-keeping). Have been utilising Thunderbird as of late though, too lazy to do otherwise (my honest reason).
So, after seeing the comment below, this appears to be an issue that is misinterpreted. The problem appears to be that mistakenly, have thought that your issue was one of an upgrade loss of your inbox, whereas the issue is that youa re actually still receiving email, however, the emails are not showing within your inbox in Evolution.
If this is correct so far, then perhaps the issue is within a setting in your Server Settings for the email account itself. Knowing how those settings are set may shed some light on the issue here; could you perhaps share that information (without revealing any specific personally identifiable information) so that a determination could be made to determine if the incoming emails are being sent to the correct folders?
That might prove beneficial.