エラーメッセージ

Devise::Mailer#reset_password_instructions: processed outbound mail in 1652.5ms
Sent mail to to@gmail.com (829.3ms)
Date: Tue, 21 Aug 2018 23:21:53 +0900
From: gmail@gmail.com
Reply-To: gmail@gmail.com
To: to@gmail.com
Message-ID: <5b7c200177d33_f7633XXXXXXXXXXXXXXX@userAir.mail>
Subject: =?UTF-8?Q?=E3=83=91=E3=82=B9=E3=83=AF=E3=83=BC=E3=83=89=E3=81=AE=E5=86=8D=E8=A8=AD=E5=AE=9A=E3=81=AB=E3=81=A4=E3=81=84=E3=81=A6?=
Mime-Version: 1.0
Content-Type: text/html;
 charset=UTF-8
Content-Transfer-Encoding: quoted-printable

<p>=E3=81=93=E3=82=93=E3=81=AB=E3=81=A1=E3=81=AF=EF=BC=81=E7=AE=A1=E7=90=86=
=E4=BA=BA=E3=81=AE=E3=81=94=E3=81=84=E3=81=A7=E3=81=99( =CB=98=CF=89=CB=98=
 )</p>

<p>Someone has requested a link to change your password. You can do this =
through the link below.</p>

<p><a href=3D"http://localhost:3000/users/password/edit?reset_password_to=
ken=3DV-vPcs3UL-8VGzQe4s_y">=E3=83=91=E3=82=B9=E3=83=AF=E3=83=BC=E3=83=89=
=E5=A4=89=E6=9B=B4</a></p>

<p>If you didn't request this, please ignore this email.</p>
<p>Your password won't change until you access the link above and create =
a new one.</p>


Completed 500 Internal Server Error in 3737ms (ActiveRecord: 10.2ms)


  
EOFError (end of file reached):

概要
Devise gem のメール発信で起きるエラーです。
※以前までは発信できていました。

・エラー発生は開発環境（localhost:3000）
・SMTP:gmail

心当たり
development.rbの設定をいじいじしたのが原因かと思います。
（確証もなくいじいじしてしまい反省しています）

何か問題・気づき・アドバイス等ありましたらよろしくお願いいたします。

Rails.application.configure do
  # Settings specified here will take precedence over those in config/application.rb.

  # In the development environment your application's code is reloaded on
  # every request. This slows down response time but is perfect for development
  # since you don't have to restart the web server when you make code changes.
  config.cache_classes = false

  # Do not eager load code on boot.
  config.eager_load = false

  # Show full error reports.
  config.consider_all_requests_local = true

  # Enable/disable caching. By default caching is disabled.
  # Run rails dev:cache to toggle caching.
  if Rails.root.join('tmp', 'caching-dev.txt').exist?
    config.action_controller.perform_caching = true

    config.cache_store = :memory_store
    config.public_file_server.headers = {
      'Cache-Control' => "public, max-age=#{2.days.to_i}"
    }
  else
    config.action_controller.perform_caching = false

    config.cache_store = :null_store
  end

  # Store uploaded files on the local file system (see config/storage.yml for options)
  config.active_storage.service = :local

  # Don't care if the mailer can't send.
  config.action_mailer.raise_delivery_errors = false

  config.action_mailer.perform_caching = false

  # Print deprecation notices to the Rails logger.
  config.active_support.deprecation = :log

  # Raise an error on page load if there are pending migrations.
  config.active_record.migration_error = :page_load

  # Highlight code that triggered database queries in logs.
  config.active_record.verbose_query_logs = true

  # Debug mode disables concatenation and preprocessing of assets.
  # This option may cause significant delays in view rendering with a large
  # number of complex assets.
  config.assets.debug = true

  # Suppress logger output for asset requests.
  config.assets.quiet = true

  # Raises error for missing translations
  # config.action_view.raise_on_missing_translations = true

  # Use an evented file watcher to asynchronously detect changes in source code,
  # routes, locales, etc. This feature depends on the listen gem.
  config.file_watcher = ActiveSupport::EventedFileUpdateChecker

  # mailer setting
    # mailer setting
    
    config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }
   
    config.action_mailer.raise_delivery_errors = true
    config.action_mailer.delivery_method = :smtp
    
    config.action_mailer.smtp_settings = {
      :enable_starttls_auto => true,
      :address => "smtp.gmail.com",
      :port => 587,
      :domain => ENV['gmail_dmain'],
      :user_name => ENV['gmail_username'], 
      :password => ENV['gmail_password'], 
      :authentication => 'login'
  }
end

行動規範の内容に同意します

回答2件

config.action_mailer.smtp_settings = {
~
:domain => ENV['gmail_dmain'],
~
}

ドメイン名に問題があると同様の結果になるようです。
Devise mailer EOF error

リンク中の下記辺りが今回の原因に該当する内容な気がします。

My config/secrets.yml isn't tracked in git (naturally) and looks like it was overwritten and lost those entries for the email provider smtp/username/password at some point.

僕も以前作成したアプリでdeviseを絡めたメール送信を行なっていたので書いておきます。

config.action_mailer.default_url_options = { host: "localhost:3000"}
config.action_mailer.raise_delivery_errors = true
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  :address => "smtp.gmail.com",
  :port => 587,
  :user_name => ENV['MAIL_ADDRESS'],
  :password => ENV['MAIL_PASSWORD'],
  :authentication => :plain,
  :enable_starttls_auto => true
}

config.action_mailer.perform_caching = false

細々したところに差異がありますが、:domainを設定せずとも動いていました。
設定する必要があるのであれば、別のドメイン名を試してみてください。