Emails sometimes take more than 5 minutes to go out, even get transfer delayed on many of them.. We have 3meg download and 2 upload... 5 people... this starting happening when we updated the version from 2014 to 2014 r2 hp1.
NSLookup will just tell you how quickly your DNS servers are responding.
My personal opinion - you have a network issue. Perhaps from your GWIA server use telnet to open a connection to mail.bonavel.com and manually enter the smtp commands to send a mail to a known address there, obviously alerting the recipient ahead of time. That will indicate, usually, if SMTP is working between the systems and whether the problem is network related or GWIA related.
Further to what I've said above, I've done some testing against that domain using mxtoolbox.com - I'm not getting an SMTP response from the server. Not too sure if that is indicating an issue on that site or not, but I suspect so.
Carry on digging and we'll get to the bottom of this together.