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.

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

• Prevents you from data breaches and liability risks
• Upgrade at your own pace
• Works with modern Rubies

Read more Show archive.org snapshot

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TMail is no longer maintained.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'