Recently I see a lot of domains with messed up MX records after they migrate to O365.
Typically they still have their old MX records with prio 10,20,30 etc and their new O365 one with prio 0.
This shouldn't be a big issue but for some reason EFA prefers all other prio MX records before considering the prio 0 one, resulting in unneeded bounces.
I can work around this by manually entering a route for each domain with this issue but it'd be nice if EFA would handle prio 0 MX records the right way.