It’s a little unclear as to exactly what you’re looking for here. But I’m going to assume that you have two email addresses and want to configure them so you can use just one.
There are a couple of different approaches and I’ll give you some examples.
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Let’s assume you have two email accounts: email@example.com and firstname.lastname@example.org.
Set up an auto forward
On one account, set up an auto-forward to the other.
When you get email at email@example.com you instruct that email service to automatically forward it to firstname.lastname@example.org. (The specifics of how to set it up will vary depending on the specific email provider you’re actually using.)
That way you can do all of your management using your Gmail account, you only need to login to that second service and all of your email appears there.
The other approach is to instruct the main account (email@example.com in our scenario) to automatically pick up the email from the other by performing a POP3 or an IMAP email fetch.
In our example we might instruct the Gmail account to perform a POP3 pickup of email from the Hotmail (aka Outlook.com) account], and once again all the email would appear in the single Gmail account.
What I do
Exactly how you do this and whether it’s an option or not depends on the specific email providers that you are using. Hotmail and Gmail actually both do this and you can work in either direction. Exactly how you set it up depends on the service; not all services provide both of these kinds of functions.
As a final example, I’ll simply point out that I have a Hotmail address. It’s now accessible through Outlook.com, but I almost never go there. Instead, my Hotmail email is automatically forwarded to my real email address where I manage it. I can read all of my email in one place, including that which comes into Hotmail and I can send email that is from my Hotmail email address as well.
Remember to login to both
If you successfully get something like this set up, please be sure to login to both accounts periodically. The problem is if that you never login to the first account because you’re managing all of your email with the second one, then the first account provider may deem your account as being abandoned. They may end up closing your account because you never login to it. So, make sure to login, say once a month, to all of the email accounts that you care about so you don’t lose them.