One thought occurred to me today; There used to be the Mailbus and Message Router products
running on VMS. They could be used to route messages between different mail systems. They
were used at DEC until about late 90's. The addresses used the format
user.name(a)*.mts.dec.com.
Unfortunately I don't remember much of them anymore, but just thought that they could
be used as a message gateway between HECnet and the Internet. If I remember correctly, the
VMS Hobbyist licenses included those.
If someone has recent experience of those, he could possibly tell more exact details about
the feasibility.
In my mind, using PMDF would be a better choice for SMTP<->DECnet connectivity. I
believe that it's available from Process Software through their hobbyist program.
Of course, I'm one of the former PMDF developers, so I'm biased. :-)
--Marc