I've seen this suggested before, that gmail and yahoo have more traffic so its fine. This is utter nonsense.
Gmail, Yahoo, MSN, Hotmail, Facebook and soon Twitter and Skype all have backdoors the feds can go and snoop when they want.
They also have very advanced filters to track mail and intercept it. Don't believe me? Have you ever seen those ads that pop up on the right hand side of gmail about what you've been talking about in your mail? Gmail has also recently changed its privacy settings to suit marketing/LE.
You cannot "blend in". Those email clients and social media outlets are the most heavily tracked.
I've said this above and got more than one PM on what I suggest so here goes:
Use a VPN.
Choose one that is located OUTSIDE of the USA and NEVER use USA servers. There are hundreds if not thousands of VPM providers.
Make sure they dont log traffic.
Make sure they do not log connecting IPs. If they do, see how long they hold them for.
You can pay for a VPN (most) with a pre-paid CC in a fake name.
Use a VPN provider that does not co-operate with your jurisdiction. Or if they do, the logs or connecting IPs are deleted in X time frame so a court order means nothing.
Use foreign email cleints situated OUTSIDE your jurisdiction. Russies, China are good ones as they will tell the USA to fuck off if they ask for Joe Black's email communication.
Change your email client every 1-2 months.
If you want, you can chain proxies, VPN's, TOR, but now your getting complicated and unless your on the run, not needed.
Some of the above is not needed for everyone, but I wish to stay "Anonymous" online and dont want no LEA's snooping my traffic.
More steps...
Only use disposable (change every 4 weeks) pre-paid phones to access the internet. Paid for with a prepaid CC (fake name) in a metro area only.
Or only access the internet using public WIFI.
Make fake Facebook, Twitter accounts with your emails as you go along. Got to leave some rabbit holes!
Google yourself and your fake accounts so you know they're available for all to see.