I’m shocked by how many people aren’t aware of this feature on Gmail.
To setup additional e-mails, just go to the Accounts section under Settings and add all of your other emails.
For example…whenever someone sends an e-mail to my bryn [at] bryn.me e-mail, I have it set to automatically forward it to my gmail. I can respond to the e-mail and select my “bryn [at] bryn.me” account as the sender.
I do this with all of my email accounts, this way I only login to one e-mail and can manage them all from one place.
Another thing I do to organize them all is to setup labels for each account so when they come in I can see what e-mail it was sent too.
The best part is that Gmail is of course web based, so no bullshit like Outlook if you’re not on you’re PC and need to check your email.