Postal is described as 'The open source mail server - Postal is a complete and fully featured mail server for use by websites & web servers. Think Sendgrid, Mailgun or Postmark but open source and ready for you to run on your own servers'. There are five alternatives to Postal for Online / Web-based, Self-Hosted solutions and SaaS. The best alternative is Sendgrid, which is free. Other great apps like Postal are Mailgun (Paid), Postmark (Paid), Mailwhale (Free, Open Source) and Cuttlefish.io (Free, Open Source).
Scalable email delivery for transactional email SendGrid transparently handles DKIM, SPF, ISP rate limits, content inspection, bounces, and spam reports to help users achieve higher deliverability. SendGrid integrates in minutes by just changing three SMTP settings.