I have a client that wants me to do a mail out campaign, but they are not hosted with me... they are hosted elsewhere.
What steps do I need to take to allow me to send a campaign on their behalf?
There is no extra work involved really - when you create a campaign you just use their domain as the 'from' and 'reply to' addresses, and we will add those in the email header.
Authentication is optional - you can send without it. If you do wish to use it for a particular client, then you need to add the DNS records to their domains DNS in exactly the same way you can do for campaigns sent for yourself.