Recently my core ceased to send any e-mails, I have tried to use SMTP Authentication module which worked briefly but now that seems to have stopped working. We initially had problems sending to corporate -email accounts (e.g. MrX@Publicservice.net) and could send e-mails (test and account-related) to civilian address (e.g. toddunctionious4@hotmail.com) but today neither are working. We are using multiple registration profiles for individual roles (e.g. client Profile for client user role) but these have not affected my other sites.