An email alias refers to an email address that is associated with another destination email address. It enables using the email alias rather than the complete or original email address to send email to the corresponding email address or the recipient. We can say that an email alias is simply a forwarding email address. The mail server simply forwards email messages addressed to an email alias on to another, the specified email address.

When you add another address to your Gmail account for sending messages, Gmail typically treats the new address the same as your original Gmail address. The added address is an alias for your original address.

You should uncheck Treat as an alias only if you run into issues when sending email from your Gmail account to your alternate (“send as”) account.

Here are some detailed explanations…

Gmail has a notion of “me” as a sender and recipient, which is why searching your mail “from:me” works.

If you choose Treat as an alias Gmail will treat the other address as “me” in addition to your main Gmail address. If you untick Treat as an alias then it won’t. Before this feature was added, all “send mail as” email addresses were treated as aliases, or in other words, treated as “me.”

There are a few minor repercussions. For example, if you send a message to “me”, Gmail will put the message into your inbox. So if you send a message to address B, then Gmail will put it in the inbox if B is treated as an alias, but will not put it in the inbox if B is not treated as an alias.

“Treat as an alias” does not affect whether your other address shows in the headers; that feature is controlled by your choice to use an SMTP server for the other address. It will also not affect specific searches for the other address, or your default reply address (e.g. “Reply from the same address the message was sent to”).

You should use Treat as an alias if the other address represents your own personal identity. You should not use Treat as an alias if the other address represents another person (such as your boss) or a mailing list.

In other words, you have a choice between the following two options:

  1. This is my address
  2. I am sending email on somebody else’s behalf

By saying treat as an alias you’re telling Google that it’s your own address. The consequences of this choice are only minor.

  1. If you’ve told Google it is your address, then if you put this address as a “To” (or Cc, Bcc) address, Gmail will simply place a copy directly into your inbox.
  2. If you’ve told Google it isn’t your address (ie, it’s not an alias), then if you put this address as a “To” (or Cc, Bcc) address, Gmail will still send the email as outgoing mail, treating it no differently to any other address.