*   Instrument the generation of Action Mailer messages. The time it takes to
    generate a message is written to the log.

    *Daniel Schierbeck*

*   Invoke mailer defaults as procs only if they are procs, do not convert with
    `to_proc`. That an object is convertible to a proc does not mean it's meant
    to be always used as a proc.

    Fixes #11533.

    *Alex Tsukernik*

Please check [4-0-stable](https://github.com/rails/rails/blob/4-0-stable/actionmailer/CHANGELOG.md) for previous changes.