Message Not Delivered

Message Not Delivered error with alias in Gmail 'send mail as' feature.

In Gmail,

• if you use the 'send mail as' feature to send mail from a different address (account),
• and you have recommended two-factor authentication (aka 2-Step Verification) enabled,

then you may eventually get this Message not delivered error:

Unfortunately, the error message is written in confusing technical jargon that makes it hard for ordinary people to figure out (a) what's causing the problem and (b) how to correct it.

The clue is the "out of date" phrase. To use 'send mail as' when two-factor authentication is enabled, you have to create and use an app password in order to setup mail sending. What Google doesn't tell you is that there's an expiration on that setup, after which it will no longer work, resulting in the above error message.

It would be much better if Google alerted you to the expiration with instructions on how to renew it, but alas, you have to figure it all out for yourself. Here's how to fix the problem when it occurs:

1. In Settings > Accounts and Import, find that 'Send email as' address and Delete it.
2. Then use 'Add another email address' to add that same email address back.
1. Leave 'Treat as an alias' checked, and click Next Step.
2. Username is your Gmail address (since you are using a Google SMTP server).
3. Password can be the same app password as before, or a new app password.
4. Then Add Account, and follow instructions to Verify it.

Voila!

References:

1. Sign in with App Passwords - Gmail Help
2. Google 2-Step Verification
3. Protect your account with 2-Step Verification - Computer - Google Account Help