I set up a test user 'test' with email email@example.com. 'test' is in the EMAIL_CONTRIBUTORS group and also has writing access to the folder the mail is sent to.
Anyway, when I send an email from the above account, mail delivery fails: "554 The user 'anonymous' is not in the email contributor group". Somehow Alfresco doesn't recognize the email address as one of a known user. When I assign the same address firstname.lastname@example.org to user admin, everything works fine.
alfresco-global.properties looks like this:
### INBOUND SMTP ###
Any ideas? It seems to work for everybody else...