Save the verification email you get when you originally set up the "send mail as". At the bottom of the message is a different link and text that says:
If you accidentally clicked the link [to verify your address], but you do not want to allow [redacted]@gmail.com to send messages using your address, click this link to cancel this verification:
If you ...
These used (?) to be handled for Facebook applications as a proxy so users do not have to release their email address publicly. Facebook assigns one when granting access to an application and the user requests an anonymous email instead. A temporary one is randomly generated for the user to receive emails from the application developer.
I also had a similar problem with them and found although you own these addresses you would have to add them individually to be able to send a reply. I couldn't find no instructions to do this but I managed to figure it out by:
Logging in to Gmail via my desktop/computer
Click on settings (gear icon)
Accounts and imports setting
Scroll to section 'send ...
I could not find a straightforward solution myself either because the plus (+) sign is reserved as a special character for "Results that match a word exactly". So I use the following alternative.
A decent alternative
Using the above search terms I can easily find anything but.
So, this is not the correct answer, but the "Check email from another account" seems to be a workaround for now.
And, to make it similar to the "send mail as" importer, I've chosen to check the first two options:
Leave a copy of retrieved message on the server.
Always use a secure connection (SSL) when retrieving mail.
This is presumably "Office 365". As far as I known, the "Send Mail As" functionality is the only way to be able to send from Gmail using another address (at least in personal Gmail). In which case, you'll need the SMTP details, as for any external email account.
POP and IMAP settings for Outlook Office 365 for business.
However, this sounds ...
So I found https://webapps.stackexchange.com/a/42968/24693 which I think solves my problem. I had a filter
never send to spam
I changed it to:
never send to spam
n.b. despite me entering the new rule as above, it appears as ...
This question has been answered here, but I'm unable to close it because of the bounty.
In short, you may use Gmail's SMTP server for your alias address, as long as you have enabled two-factor authentication.
From Mail routing and delivery: Guidelines and best practices - Google Apps Help
Routing examples and uses cases
Several mail routing and delivery scenarios are listed below. If a use case applies to you, click the link for detailed instructions.
Default routing - Use split delivery to route unregistered Google Apps users to an on-premise mail server
After you create your alias through Admin Console then you have to set it up as a sender in your Gmail settings.
Gmail Mailbox > Gear icon > Settings > Accounts > Send Mail as : Add another email address that you own
You put the display name you want, you enter the alias you created through Admin Console (firstname.lastname@example.org), you leave checked the "Treat ...