Since version 2.27.1 of Email-ext, emails are triggered for "1st Failure", even though the build is successful.
Downgrading to 2.27 solves the issue.

Excerpt from the log :

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 25 minutes 49 seconds
[INFO] Finished at: Tue Mar 19 12:27:02 CET 2013
[INFO] Final Memory: 61M/317M
[INFO] ------------------------------------------------------------------------
En attente que Jenkins finisse de récupérer les données
channel stopped
Archiving artifacts
Email was triggered for: 1st Failure
Trigger Failure was overridden by another trigger and will not send an email.
Trigger Still Failing was overridden by another trigger and will not send an email.
Sending email for trigger: 1st Failure
Sending email to: **@*.**
Finished: SUCCESS