I'm using the mail gem and received this error when sending emails:

530 5.7.0 Must issue a STARTTLS command first

The fix was to first install the tlsmail gem:

gem install tlsmail

Then I had to add this to config/initializers/email.rb:

require 'tlsmail' 
Net::SMTP.enable_tls(OpenSSL::SSL::VERIFY_NONE)