This is a simple SMTP Server that can have a chain of mail processors inserted, this is not a production tool, and was written for teaching purposes.
$ go build ./cmd/john
$ ./john smtp --init-maildirMail will be delivered to ./tmp in Maildir format.
It might be simpler to use HTTP to send emails.
$ ./john http --init-maildirThis defaults to serving on :8080
You can send emails via curl
#!/bin/sh
curl -d "Testing" \
-H "John-Envelope-From: testing@example.com" \
-H "John-Envelope-To: test@example.com" \
-H "John-Mail-From: testing@example.com" \
-H "John-Mail-To: test@example.com" \
-H "John-Mail-Test-Header: this is a test" \
http://localhost:8080/Again mail will be delivered to ./tmp in Maildir format.
$ echo "This is the message body and contains the message" | mailx -v -r \
"someone@example.com" -s "This is the subject" -S smtp="localhost:2525" \
testing@example.com$ go test ./...