I needed to use an external SMTP server to get mail through to certain providers (notably, hotmail).
I was sending my action mailer email but couldn’t be sure that I’d configured the settings correctly as I didn’t get any detailed log output (or at least not where I was looking).
I dug into the code and found the lines that were responsible for sending the email. Replaying that code in the console allowed me to identify the problem immediately (a missing domain in my case). I thought I’d paste those few lines here in case they’re of some use to someone else.
message, from, to = 'test message', 'sender-email', 'recipient-email' s = ActionMailer::Base.server_settings Net::SMTP.start(s[:address], s[:port], s[:domain], s[:user_name], s[:password], s[:authentication]) do |smtp| smtp.sendmail(message, from, to) end