To create and test a monitoring plugin for Postfix, I had to force Postfix to queue some messages.

I looked at several Mock SMTP Servers but none was quite as simple as I needed.

I ended up using Postfix's own features.

Defer All Messages

postfix/main.cf

default_transport = defer

Defer Some Messages

postfix/main.cf

transport_maps = hash:/etc/postfix/transport

postfix/transport

.example.com error:
example.com defer:

sources:

• Use Postfix Transport Map & Relayhost Map For Flexible Email Delivery by Xiao Guoan
• Fake SMTP server with Python by Stuart Colville
• E-mail Testing with Mock SMTP Server by Maciek Makowski
• Postfix Manual - transport(5)
• RFC5321 Simple Mail Transfer Protocol
• smtpd – SMTP Server – Python 4 Documentation