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.
Fix for Rails if you don't have an awesome MTA
TMail is no longer maintained.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'
Posted by Henning Koch to makandra dev (2012-09-18 16:35)