I just called tech support and was told that this never should've worked, which was briefly and grimly entertaining, but ultimately did not resolve my issue.
I asked whether the tech could tell me when the NAT box (using the SMART/RG device) last got a new DHCP lease, since that also appears to have happened within the last few days and I'm curious if there's a correlation. Unfortunately, the tooling available to tech support won't expose this info.
I can telnet/nc to port 25 from my Sonic shell account, so there doesn't appear to be a connectivity problem on the host side.
I've burnt too much of my Sunday already trying to root cause this; could anyone out there please suggest a solution to make this start working again, or a way to contact someone in Sonic netops who has tooling to answer some of my questions?