Unable to send email from one domain hosted on Plesk to another domain hosted on a Plesk server: Recipient address rejected: User unknown in virtual mailbox table

Symptoms

Domain example.com exists on a local Plesk server and on external server.

Domain resolves to globally to IP address of external server. IP address 203.0.113.2 does not belong to a Plesk server:

# dig +short example.com && dig +short mail.example.com
203.0.113.2

Mailbox user@example.com does not exist on local mail server (with Plesk). 

Mailbox user@example.com exists on external mail server with IP 203.0.113.2.

Unable to send email from one domain hosted on Plesk to another domain (user@example.com) hosted on Plesk server fails:

CONFIG_TEXT: SMTP Error (550): Failed to add recipient "user@example.com" (5.1.1 <user@example.com>: Recipient address rejected: User unknown in virtual mailbox table)

or:

CONFIG_TEXT: cannot chdir to mailname dir user: No such file or directory
Unknown user: user@example.com
NOQUEUE: reject: RCPT from localhost[::1]: 550 5.1.1 <user@example.com>: Recipient address rejected: User unknown in virtual mailbox table; from=<> to=<user@example.com> proto=ESMTP helo=

or:

CONFIG_TEXT: Reason: The recipient does not exist
Message could not be delivered - the address was not found, is unknown, or is not receiving messages.

Postfix mail server is used.

Cause

Postfix connects to a recipient's mail server mail.example.com locally without looking for external mail server.  Next, Postfix cannot find the mailbox user@example.com on a local mail server (with Plesk), thus, the message is rejected. Postfix does not retry to deliver email to the external server if recipient mailbox was not found locally: this is Postfix's behavior by design.

Resolution

As a workaround disable mail service for domain example.com domain hosted on Plesk:

1. Log into Plesk UI.

2. Open the tab Domains > example.com > Mail Settings and unckeck option Activate mail service on this domain.

3. Click OK.