After briefly looking at your website, I think that mailinator.com looks to have superior features.
1) You don't even need to go to Mailinator to get an email address. You just know that any time you have any address you want on hand.
2) With Mailinator you just use any name when you register on some website. (generated names are suspicious)
3) Mailinator has different domains to avoid blacklisting (domain name doesn't matter. You just need to know the username to check your mail)
4) There are websites that try to you using Mailinator by trying to log is with the name of your e-mail. For these services you can use another generated name that can only be used for sending mail, not for logging in.
5) Mailinator does not retain your messages for long. Also everything is stored in memory (as far as I heard), so the messages cannot be stolen.
Mailinator has an interesting approach to solving the problem , to my taste it's not as simple to use as Air Mail but 1 and 2 are a big plus i must admit. I will see how to add these.
not a domain re seller myself, but I do have a few short domains I use for Air Mail, as Air Mail is a disposable temporary address it doesn't really matter , if I sell any of these I will make sure they are dropped from Air Mail in an orderly fashion.
I really like your service and have used it a few times. I doubt the UX will be affected that much if you were to include a few passive ads. So I would say, go for it, make some money, you deserve to.
What are passive ads ? do you mean more GoogleAds ? I'm not in it for the quick dollar, I want to get this service to as many people as possible and be in it for the long run.
http://getairmail.com
wish I had a cent for every email it processed ;-)