Other gems and projects

Mailgun

• mailgun doesn't receive email
• pew_pew doesn't receive email
• mailgun-rails only delivers email
• mailgun_webhooks is a framework for processing Mailgun webhooks
• mailgun.rb only delivers email and isn't a gem
• mailgun_API only delivers email and isn't a gem
• mailgun-email-receiving is a Rails app, not a gem
• holster isn't implemented and isn't a gem

Mandrill

• mandrill-api is the official gem
• mandrill is a thin API wrapper
• mandrill_mailer is a substitute for ActionMailer
• mandrill-rails parses a webhook into a Mandrill::WebHook::EventDecorator

Postmark

• postmark parses a webhook into a Ruby hash
• postmark-mitt parses a webhook into a Postmark::Mitt object
• simple_postmark is superceded by postmark
• postmark-rails is very light Rails integration

SendGrid

• sendgrid_webapi for Web API
• sendgrid_toolkit exposes a subset of the Web API
• sendgrid_web is a thin wrapper around the Web API
• sendgrid_cli for Web API Spam Reports
• sendgrid_subusers for Customer Subuser API
• gatling_gun for Newsletter API
• sendgrid_postback for Event Webhook
• sendgrid_events for Event Webhook
• sendgrid_api isn't implemented and isn't a gem
• sendgrid isn't implemented

Plus tons of ActionMailer integration gems.