Dyn Email Gateway delivery to non-routable SMTP server

Sadly no, indeed if your ISP is allocating you RFC 1918 IP addresses you cannot access any services on your network remotely.

My question is more along the lines of "can my server poll/pull mail" from it's temporary storage on Dyn Email Gateway.

Cry Havok already answered correctly with "Sadly no". Or, to say it more explicitly, none of the Dyn services can do what you want to achieve. The model you're describing is any normal mail service (e.g. Gmail), i.e. an externally hosted mail server from where you can pull mail in the client role, but not with SMTP, but with POP3 or IMAP.

Thanks for the feedback. I do understand RFC 1918.

My question is more along the lines of "can my server poll/pull mail" from it's temporary storage on Dyn Email Gateway.

My understanding of smtp protocol is that normally the source opens a connection to the destination and sends.

I need the reverse. The destination opens a connection to the source and ask if any mail is waiting for it. Similar to how a POP3 connection works.