We let any customer have DKIM signed messages on their own domain, if that is what you mean. If not, can you clarify?
http://support.sendgrid.com/entries/98008-how-to-eliminate-t...
Also, do you support mass newsletters sending to subscribed users? I use sendgrid because many other providers like MailChimp said they don't allow it.
To be fair, sendgrid's service is also very good, e.g. customer support, reliability etc.
We do not allow newsletter (or any bulk style) sending - I explained our reasoning behind separating and only sending transactional email through Postmark - by definition of transactional, the recipient is expecting the email and it is a custom response to an action.
Edit: typo.