Explain version constraints

actionmailer/lib/action_mailer/base.rb

@@ -402,7 +402,8 @@ module ActionMailer
   #     This is a symbol and one of <tt>:plain</tt> (will send the password Base64 encoded), <tt>:login</tt> (will
   #     send the password Base64 encoded) or <tt>:cram_md5</tt> (combines a Challenge/Response mechanism to exchange
   #     information and a cryptographic Message Digest 5 algorithm to hash important information)
-  #   * <tt>:enable_starttls</tt> - Use STARTTLS when connecting to your SMTP server and fail if unsupported. Defaults to <tt>false</tt>.
+  #   * <tt>:enable_starttls</tt> - Use STARTTLS when connecting to your SMTP server and fail if unsupported. Defaults
+  #     to <tt>false</tt>. Requires at least version 2.7 of the Mail gem.
   #   * <tt>:enable_starttls_auto</tt> - Detects if STARTTLS is enabled in your SMTP server and starts
   #     to use it. Defaults to <tt>true</tt>.
   #   * <tt>:openssl_verify_mode</tt> - When using TLS, you can set how OpenSSL checks the certificate. This is

guides/source/action_mailer_basics.md

@@ -862,6 +862,8 @@ config.action_mailer.smtp_settings = {
   read_timeout:    5 }
 ```
 
+If you are using an old version of the Mail gem (2.6.x or earlier), use `enable_starttls_auto` instead of `enable_starttls`.
+
 NOTE: Google [blocks sign-ins](https://support.google.com/accounts/answer/6010255) from apps it deems less secure.
 You can change your Gmail settings [here](https://www.google.com/settings/security/lesssecureapps) to allow the attempts. If your Gmail account has 2-factor authentication enabled,
 then you will need to set an [app password](https://myaccount.google.com/apppasswords) and use that instead of your regular password.