Collecting Gmail-based email from Hotmail
In this scenario, Hotmail will be the main email account and Gmail is the secondary account. You will use Hotmail to collect email that was originally delivered to Gmail.
1) To collect your Gmail from Hotmail, you'll need to first ensure that POP3 access is enabled in Gmail. At the time of this writing, you configure this setting from Options | Mail Settings | Forwarding and POP/IMAP | POP Download from within the Gmail web interface.
You have two settings to configure here. First, you'll need to enable the POP protocol, and you have two choices, which amount to "for all mail" (i.e. include all of the email that was previously delivered to Gmail) or "for mail that arrives from now on" (i.e. excluding all of the previously delivered email). After choosing one of those methods, you'll need to determine what Gmail does with email that is collected from the primary (Hotmail) account. There are four choices: Keep Gmail's copy in the inbox, mark Gmail's copy as read, archive Gmail's copy, or delete Gmail's copy.
Looking at my own needs, I would elect to only deliver new messages from Gmail to Hotmail because of the absurd amount of email I've received over the years. (Though I will be looking separately at backing up my Gmail-based email archives later.) And I would choose to archive Gmail's copy, since that will ensure I have a second copy of any Gmail-based mail, and it won't clog up the Gmail inbox. Your needs may vary.
Whatever you choose, click the Save Changes button at the bottom of the page when you're done.
2)In a separate browser window or tab, logon to your Hotmail account. Select Options | More Options | and then Sending/receiving email from other accounts (under the section Managing your account). Click the link titled Add an email account.
In the provided form, enter your Gmail address and password and click Next. (There is an Advanced options link that will let you configure server settings manually, but this isn't necessary for Gmail.)
Hotmail will attempt to configure the account. When it's done, you'll be presented with a second page in which you can configure where Gmail-based email will be delivered. You choose from existing folders (like the Inbox, which I personally prefer) or create your own folder. You can also colorize these Gmail-based emails so that they are visually distinct from Hotmail-based email.
When you're done configuring these options, click Save. Hotmail will configure the email collection and then return you to the Hotmail inbox.
3) There's one more step to endure before this email management solution is complete: You need to reply to a verification email that Hotmail sent to your Gmail account. This ensures that you're actually who you say you are, of course. If you return to Gmail, you will see (or will soon see) an email titled Windows Live Hotmail: Verify your email address. Open this and click the provided link. Once you do so, Gmail will allow Hotmail to collect your email.
You may also want to revisit the Sending/receiving email from other accounts area in Hotmail Options. (Remember, it's in Options | More Options | and then Sending/receiving email from other accounts). Now, you'll see that Hotmail is configured to collect email from Gmail. You could also choose to collect email from more accounts or, if you wish, to remove the Gmail account you just attached.
Also, when you send email from the Hotmail web interface, the From address has a drop-down symbol next to it, indicating that you can change from "From" address, on the fly, to any configured account. In this case, that will include your main Hotmail account as well as your secondary Gmail account.
Note: There is one limitation to this approach. When you send email from Hotmail as if it were from Gmail, Hotmail will note in the From information that it was sent from your Hotmail account on the behalf of your Gmail account. This may not be how you want that to read, but to my knowledge there is no way to suppress that note.