Most email is SPAM. You can tell the SPAM by looking at the content in many cases, but as well all know that is not 100% reliable.
However, if you are grabbing only the communications data, i.e. the "from", "to" and date/time, you cannot tell the SPAM.
So most of the data you collect is bogus. The email addresses are mostly not even real, as sender or recipient, and the apparent associations between people are not real either.
It is hard to see how this data can be useful even just for investigating connections between people, and clear that it cannot be trusted as evidence of anything.
Of course, if you then get some sort of packet snooping systems in place you find that you can be collecting a lot more data.
It would be trivial for someone to make an application that connected (apparently) to a mail server and apparently tried sending emails from lots of email addresses to lots of destinations. The email would not be real. The mail server would not be real. The data would just be the headers to catch the snooping systems.
This would be low levels of upload on a broadband line, constantly, 24 hours a day. It could even stop when you are using the Internet so as not to disrupt real usage.
It would make the data collected even less useful. It would simply poison the database that GCHQ are collecting.
So, who knew, SPAM may have some use after all!