Allow to customize FROM header
We should distinct the FROM of the envelope and the FROM of the header. The FROM of the envelope has a more technical usage because it is checked by the MTA and it is used to send notification about failure to delivery. The FROM of the header is for the reader, the MUA will use it for the reply. The envelope one as technical should only be set by the configuration. The header one can be defined by the user and fallback to the envelope. Also in module for which we allow the user to define the FROM header, we should allow to define one address per language. Because on multi-lingual companies, the user will want to use different mailboxes per language of reply. With this requirement it is clear that we must distinct the two kinds of FROM.