On Sat, 2 May 2015 12:36:05 +0000 (UTC), Maurice wrote:

Anyone who is using such an approach (like me!) should be aware of
the increasing use of the SPF protocol by ISP's and senders that
is aimed at avoiding the use of email forwarding.

(Not sure that's the point of SPF!)
So you need to use a (separate from your ISP*) mail service provider
that handles SPF, perhaps by re-writing the envelope header stuff
using the Sender Rewriting Scheme
(https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme).
[*]I was howled down before for suggesting using an email service
provider, hopefully it won't happen this time!