Question

What is the difference between “deliver” and “deliver!” methods of the mailer? (specifically for rails 3.1)

Answer 1

An ActionMailer method can be triggered using one of two methods:
either deliver or deliver! – the main difference between the two is
that the second will throw exceptions if it cannot be sent, which is
why I tend to prefer using it. Something to keep in mind though, is
that using deliver! will call any registered Mail Observers, but not
Interceptors – meaning that your mail will be sent unaltered.

Taken from this blog.