Built-in Postfix in its default configuration accepts only emails addressed to it as an SMTP server e.g number@smseagle_ipand by default rejects emails sent to different domains (so it can't be used for email spoofing) but it can be configured to relay them further if necessary.
To do so, please connect with SSH (as root) to your SMSEagle device and in the file /etc/postfix/main.cf please modify parameter#relay_domains = $mydestination
to:relay_domains = $mydestination, yourdomaintoberelayed.com
Then emails to address@yourdomaintoberelayed.com would be forwarded. If you need more domains, just add them in the same line, separating with comma:relay_domains = $mydestination, yourdomaintoberelayed.com, yourdomaintoberelayed.pl
Optionally it also may be necessary to change:mynetworks_style = host
tomynetworks_style = subnet
but this you will need to check if it's required in your environment.
After changes please restart the Postfix service:service postfix restart