Hello Etienne, this appears to have failed due to the length of the SPF record. The SPF RFC standard states SPF records should be limited in length to prevent DNS timeouts during SPF lookups. The recommendation is to set SPF PERMERROR to 0 in your Settings>Antispam>Spam Checks to ensure users on these domains can deliver mail to your server if you deal with a lot of domains with nested SPF records like this one.
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com