This is particularly useful for players who have a normal OOC gmail account and don't want to switch between IC and OOC emails.
In gmail: under the "settings" option up top, go to the 'account' tab. The third option down is "Get Mail" - fill that out for your character's email address, and emails from that account will be sent to your main account. Then go to the top option, "Send Mail". After filling that out, you can write emails from your main account as if you were logged into the character's email, with the character's address being the sender. When you reply to an email sent to your character's account after opening it in your main account, you will reply as the character's email, since it was sent to the character's email.
Just make sure you send from the correct account when sending Nexus emails!