I see there's an option to allow emails to be sent from (or at least appear to come from) a custom domail (I assume my clients domain?). Would this require setting up for each client that I (hopefully) sign up? If there's a problem doing this - say, I can't get access to their account settings - do the emails appear to come from my account? For example (if my account is 'webmailer') webmailer.createsend.com
The from address is always configurable on a per campaign basis, so your clients can always make it seem like the email is coming from them.
There is also the option for a custom domain. That's account specific and would be the same for all of your clients. It's used as your site address and also used in tracking urls (for all client campaigns). That wouldn't have anything to do with making the email seem like it's coming from your client's though, it's just a way of customizing your account.
Lastly, there is also the option to set up email authentication. This would need to be set up in the DNS for each of your clients. Is basically 2 different methods of telling receiving servers that our servers are indeed authorized to send mail for that clients domain.