Postmark App (http://postmarkapp.com) has worked well for me in terms of sending large quantities of emails in the past. Easy integration, good team, active development.
Does sendgrid allow this? I've signed up to SocketLabs which seems to do it but haven't been able to get SMTP sending working correctly yet, haven't spent much time on it though.
For the record, the amount of hassles I've had with SMTP servers these services are very handy, especially for logs etc like postmark has