Mail app opening by itself after installing El Capitan

Hello,

After installing OS X El Capitan (version 10.11) on both my iMac and MacBook Air, every time I wake or startup (even when not choosing to reopen apps while turning off the computer) my computers, the mail app (version 9.0) automatically opens. I don't care about my emails enough to check everyone and this is getting really annoying. I cannot find any way to make the mail app stop automatically opening after waking or starting up my computers. Is there anyway to make this stop? This never happened on OS X Yosemite. This is the only issue I'm having with El Capitan, so having this fixed would be great.