The ActionMailer in Rails 2 depends on a buggy version of TMail Show archive.org snapshot , which sometimes inserts a blank line into the mail header when sending a mail to multiple recipients. This makes the header end prematurely.
The reason why this is not exploding in your face all the time is that when you are relaying your e-mail through an MTA like Exim, it will fix this for you.
TMail is no longer maintained. The bug is fixed in a fork, so say this in a Gemfile
:
gem 'tmail', :git => 'https://github.com/eac/tmail.git'