You may disable the SSL certificate change check. It's somewhere in the settings, in App settings-> Network, I think.
I wouldn't recommend that.
Certificates are there for a reason, not just to annoy you. Because one day you'll have a man-in-the-middle attack while on some random wi-fi network, and someone (not this
someone but someone else, a perpetrator ;-) ) will snatch your password.
Certificate verification makes sure that Aquamail is talking to the server directly, and that communication (including the login/password) is not intercepted.
Circumventing that defeats the security.
The correct approach is exactly what
someone did: to verify that the change in the certificate is legitimate. And the approve and continue using the app.
(The option to disable this should be used only in rear cases while the servers are undergoing some maintenance/transition, and only as a temporary measure.)
Update:
Someone: For more details on this issue, please read this FAQ item:
http://www.aqua-mail.com/?page_id=227'SSL certificate change', Gmail and others