Your message wasn't delivered because the address couldn't be found, or is unable to receive email.

Question

My client owns a domain on which we have set up email aliases forwarding to her personal email. There are two aliases forwarding to the same personal Gmail account. That personal Gmail account has been verified via one of those email aliases. No matter how many times I've tried to set up the second alias (forwarding to the same personal Gmail account), I cannot get it verified. I go through the process to "Send email as" just as I did for the first email alias and get to the last step, where you either click the link in the verification email or provide the verification code, but instead the email I get is:
Subject line: Delivery Status Notification (Failure)
Body: Address not found. Your message wasn't delivered to [email alias] because the address couldn't be found, or is unable to receive mail.
Error: 550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. Learn more at https://support.google.com/mail/?p=NoSuchUser b205sor719585lfd.86 - gsmtp
I have two separate app passwords set up, one for each alias, but I cannot get the verification link or code to come through. PLEASE HELP.

Keith R (The Sweeper) · Accepted Answer

Hi Natalia, the Google test site https://toolbox.googleapps.com/apps/checkmx/check?domain=dolansden.com has some suggestions that will improve email delivery for your domain, but the issue with setting up Send Mail As should be resolved as follows.
Use smtp.gmail.com as your SMTP server with the full email address as the username and the password set as below.
Since you are setting up to send as a G Suite account (Gmail with a company domain), you should enable two-step verification (2SV) in the account you are sending as, because that is the only solution that will work into the future.  This has side effects that will impact your use of the account.  See Two Step Verification (2FA).
If you have already enabled 2SV on the remote account, you will need to use an application specific password (created in the remote account) in the Send Mail As settings: Signing in using application-specific passwords
If 2SV is not enabled, you must enable less secure access: sign into the remote (G Suite) account using a web browser at https://mail.google.com, then go to All Settings > Accounts and Import > Other Google Account settings.  Under Security, scroll down and enable access for less secure apps.  This is essential for POP and SMTP connections and Send Mail As uses SMTP.  NB: Even if you have checked this before, check it now because Google disables this access if it is not in regular use.
If there is still a problem, try clearing Captcha: visit https://accounts.google.com/DisplayUnlockCaptcha and sign in with the Gmail username and password.  If necessary (it's usually not), enter the letters in the distorted picture then press Continue. This will allow ten minutes for SMTP to register as an approved connection.  NB: you must open this page using the credentials for the account you are connecting to via Send Mail As - if the browser is already signed into another account (and it will be if you are setting up Send Mail As in a Gmail account), you must sign out first.  You can make this a bit more convenient by using a different browser, not a different window, to sign into the remote account.  Also, you must trigger the Send Mail As to make the connections within ten minutes of pressing Continue.

Keith R (The Sweeper) · Accepted Answer

Hi Natalia, I first need to determine what you mean by "alias" as that word is used in a lot of different ways.
Are these aliases accounts that you can sign into on another domain or are they forwarding addresses that exist only as an email address that must be forwarded somewhere else?
The error message you are getting indicates that the email address you are trying to add doesn't yet exist at the other domain.