I didn't use to put anything in my email signatures but few weeks ago I put my site link in my signature and I was really surprised to see the analytics results because i got around 31 clicks from (mail.google.com) means those clicks were from my email signatures. If you too don't use email signatures you should put your sites in email sigs. I am sure you will get some traffic from that link as well. By the way, it depends on the number of emails you send/receive. I usually send around 10-15 emails daily.