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I'm getting sporadically bounced emails while sending from Gmail's web client using the Send-as function. The send-as account is properly setup and works normally, but occasionally fails about once a week. It's typically with attachments to multiple people.

The bounce is from authentication failure with this message:

Message not delivered
You're sending this from a different address or alias using the 'Send mail 
as' feature. The settings for your 'Send mail as' account are misconfigured 
or out of date. Check those settings and try resending.
LEARN MORE

The response from the remote server was:
334 VXNlcm5hbWU6 UGFzc3dvcmQ6 Authentication unsuccessful 
[BL0PR02CA0045.namprd02.prod.outlook.com]

Clearly there's an authentication issue, and it's likely from the outlook.com side. But it's sporadic.

Emails with attachments went through just fine immediately before the bounce and the same email works fine if I wait a few hours.

Google Support is not helpful. They recommended I enable "Less Secure Apps".

Reducing security is not a good idea.

What's going on? How do I troubleshoot or find an alternative to send using the outlook servers but from Gmail web client?

Using Outlook as a daily driver will be terrible. There must be a solution besides Outlook or OWA web client.

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