QMail unable to send email to gmail.com or any address outside the network

One way to handle this would be to route the message on the head server to a .qmail file and then in the .qmail file forward the message to user@mailserver1.example.com and user@mailserver2.example.com. I think if you use the 'forward' program you could create a generic catch all that forwards all messages to the two servers.

/var/qmail/aliases/.qmail-default might look like this:

|forward "$LOCAL@mailserver1.example.com" "$LOCAL@mailserver2.example.com"

For emails, a domain is not merely an alias or human readable form for an IP address: mail exchanger MX records exists to specify mail servers responsible for accepting email messages on behalf of a recipient's domain. There could be several servers accepting mail for the domain, and they are not necessarily on the same IP that's in the A record for the domain. A mail system can have several servers: incoming servers might be separated from outgoing servers and mail storage servers etc. The A record is only used when there's no MX records specified for the hostname.

However, there's no (other) limit in email address format that you could not send emails directly to <user@hostname.example.com> or even <user@[198.51.100.10]> (IP with the square brackets). If there was a mailserver that accepts email using the plain hostname or even IP address, it would do. But what you are suggesting doesn't work globally in practice:

• Most email systems have several domains and need to handle email separately for them all. The username itself may not have been bound to any actual mailbox as <user@example.com> might be a different person than <user@example.net>
• While this was common couple of decades ago, fighting spam has made things more complicated and accepting email has strict limits.
• The usage of SMTP port 25 is very limited on consumer grade internet connections due to abuse (spambots). There isn't really that much use of SMTP for IoT devices.