RFC compliance.
RFC compliance.

Looking at the grammar, I don't see any reference of such a
syntax but there are a lot of similar examples into the RFC so
I guess it is allowed...

fixes #453

carragom/modoboa,mehulsbhatt/modoboa,bearstech/modoboa,tonioo/modoboa,RavenB/modoboa,bearstech/modoboa,RavenB/modoboa,mehulsbhatt/modoboa,modoboa/modoboa,carragom/modoboa,mehulsbhatt/modoboa,tonioo/modoboa,modoboa/modoboa,bearstech/modoboa,modoboa/modoboa,modoboa/modoboa,bearstech/modoboa,tonioo/modoboa,RavenB/modoboa,carragom/modoboa