Invitations, announcements, and why sometimes no email goes out at all.
Almost every provider rejects a message if the sender address is not the same one the site authenticates with on the mail server. It is what stops anyone writing on behalf of somebody else.
When it happens, not a single email goes out, and the error the server returns names neither of the two addresses, so it is impossible to guess. That is why SociWeb checks it beforehand: if they do not match, a red warning appears at the top of the Invitations screen with both addresses and where each one is changed.
It is fixed in System → Global Configuration → Server tab → Mail, by making the sender and the SMTP username match.
Magón panel → Invitations. You can filter by section and leave out under-18s. Each member gets a link to set their own password; the link expires, and the same screen resends it to anyone who has not used theirs.
Announcements is for sending to a group. Messages is for talking to one member, and works as a conversation: they reply from their own area and the thread is not lost.
Magón panel → Notices is the noticeboard: it appears in the members area without sending any email. Exactly right for what does not deserve an email to two hundred people.