One thing you can do in instance where you're waiting for propagation is forcing a /flushdns on the SmarterMail server using ipconfig /flushdns which should cause us to get the updated DNS record the next time we check. This won't help in scenarios where the DKIM record is unreachable via DNS however and for that I recommend setting primary/secondary DNS (in Settings>General) to a known good DNS host temporarily such as:
1.1.1.1 (Cloudflare DNS)
8.8.8.8 (Google DNS)
Kyle Kerst
Lead Internal Network/System Administrator
SmarterTools Inc.
smartertools.com