This is too weird!
I just noticed and confirmed through comparing outbox & inbox that when I send a message that
A) is FROM an SG address, and
B) has my actual (registered with SG) address in the BODY of the email:
somehow, somewhere in transit, the "actual" address in the BODY of the email is being changed to the SG address the mail was to have been from!
After much confusion with my ISP's outsourced support desk (they of course, thought I was referring to an address that they didn't host as my actual address kept being replaced by the SG address I was sending from), I figured this out and tested it by sending an email to myself from an SG address to an SG address - but also to a non-involved work address. I included both - different - addresses in the BODY of the email.
The one I received at work looked normal.
The one I received through my webmail app didn't - the "actual" address in the body had been changed to the SG address!
One email sent to two recipients with different body text upon receipt!
Any ideas? I'm scratching my head on this one!