Hello Marc. This actually looks to be a timeout while we're performing the spam checks and RDNS checks. You can see that because they went ahead and disconnected before we responded with the RDNS check findings. To correct that you can try implementing the following DNS servers, and eliminating any RBL/URIBL lists (Settings>Antispam>RBLs/URIBLs):
1.1.1.1
8.8.8.8
With those implemented, you should find these delays clear up and these emails should then deliver successfully. Please don't hesitate to submit a ticket with us on this if you need a hand, but I think that should do it!
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com