First, it is proper Netiquette to aggregate accounts. So, do use one inbox for multiple accounts. Given, most of of us have more than one.
Aggregate Netiquette Personal Email
It follows, we combine email addresses into one whole inbox. Next, third party apps make it possible. Then, software does not require multiple accounts.
So, one account can get mail for another. Then, a third party. Given, the software provider is separate from the email provider.
Personal Email Aggregate
Thunderbird is a popular third party email software program. Free email accounts from large companies offer the service as well. It makes it convenient to have all emails in one inbox.
Aggregate accounts into a single inbox because it is saves time.