The email-ext plugin makes the PROJECT_URL environment variable available to be sent via emails. It seems to work fine for standard projects. However if the email-ext plugin is used in coordination with Promoted Builds, the actual URL value constructed by the email-ext plugin in the hudson.plugins.emailext.plugins.content.ProjectURLContent is not correct. Not sure if this is caused by the email-ext plugin or if the build number of the project is not passwed down to the email-ext correctly. The PROJECT_URL seems to be missing the actual build number in it. For example :

Good URL : http://myhost.com/hudson/job/MY_JOB_NAME/promotion/BUILDNUM/PROMOTION_PROCESSNAME/

Bad URL being generated : http://myhost.com/hudson/job/MY_JOB_NAME/promotion//PROMOTION_PROCESSNAME/

Note the actual build number represented by "BUILDNUM" is missing so that 2 slashes are used instead.