Error: Postfix - do not list domain example.com in BOTH mydestination and virtual_mailbox_domains

In my case, at least, removing mydomain.com from mydestination cleared things up. On my server, mydestination is just localhost, localhost.localdomain

GREAT SOLUTION, works for me.

Inspecting Postfix’s email queue.

1-  Display a list of queued mail

mailq

or

postqueue -p

To save the output to a text file you can run

mailq > myfile.txt

or

postqueue -p > myfile.txt

Testing SMTP, POP3 and IMAP protocol access for Postfix Mail Service

SMTP

telnet: > telnet mx1.example.com smtp
telnet: Trying 192.0.2.2...
telnet: Connected to mx1.example.com.
telnet: Escape character is '^]'.
server: 220 mx1.example.com ESMTP server ready Tue, 20 Jan 2004 22:33:36 +0200
client: HELO client.example.com
server: 250 mx1.example.com
client: MAIL from: <[e-mail]>
server: 250 Sender <[e-mail]> Ok
client: RCPT to: <[e-mail]>
server: 250 Recipient <[e-mail]> Ok
client: DATA
server: 354 Ok Send data ending with <CRLF>.<CRLF>
client: From: [e-mail]
client: To: [e-mail]
client: Subject: Test message
client: 
client: This is a test message.
client: .
server: 250 Message received: [e-mail]
client: QUIT
server: 221 mx1.example.com ESMTP server closing connection