Email using Outlook when overseas

[#1]

All domains are hosted by the same entity with the exception of my main address which is an alias based in New York. 
All email services were 100% operational when I was connected to PN in the UK.  It only started to go wrong when I tried to send traffic from Mexico.
As I said the problem with the .co.uk domain is not really that serious.  It has just become a challenge.  It is like moving from 100% service to 98% service and it may not be worth fighting over.  I certainly don't want to trouble others on that front unless my curiosity is shared.
with my thanks,

Michael

[#2]

I can only speculate since we don't know what checks the Cloudmark anti-spam appliances make when relaying mail from a non-Plusnet coonection. However I wonder if a recipient domain MX record is required and the odd-ball doesn't have one? In this case mail transport falls back to use an A record but verifying that might be too complex for a quick check.
As I said speculation but…
David

[#3]

@mhayes - for the email address you can't send with, change Outlook's outgoing server authentication to use your main Plusnet log-in details instead of whatever it's set to now.