Hello,
Completely agree with Logan. He raises a very good point about Google Analytics being good at subcategorising traffic.
There is another potential solution that may be worth considering. It requires looking at the timescale and quality of the traffic that is coming through from email.
Google uses all kinds of things as ranking factors for SEO, including site usability. If email contributes a large amount of traffic to your site, but has a high bounce rate/low pages viewed/reduced time on site, it is possible that these could be negative ranking signals to Google. This would mean that although it could be getting more traffic a page is losing ranking.
I would recommend having a look in Webmaster Tools and, under Search Analytics, have a look at Pages and Position. If you see a decrease then I would suggest looking at the quality of the traffic from email. You could even look at the conversion optimisation of your landing pages.
I would recommend this blog for a fairly in-depth look at ranking factors. If you find you want to look at Conversion Optimisation then there is a fantastic book called "Thinking Fast and Slow" by Daniel Kahneman (if you haven't already read it).
It's always hard to recommend SEO solutions but I hope this helps. If nothing else it should give you somewhere to start.
Cheers,
Luke