I think an adequate explanation for the URL-fetch mystery is that some
people
run a virus scanner or have their e-mail delivered to a mailservice that
does
scanning, and this scanner fetches all URLs in the message to check if
they
point to malware...
No amount of validation of the subscribers will solve that problem.
The only thing you can do is obfuscate the links, but that is
incovenient for
the reader.
Similar problem: whenever you download a program, a second fetch is done
by
Microsoft/Google/Apple/whatever your favorite vendor to check what it
is.
I have seen this several times on a download server we run at work.
Rob