I'm not familiar with paperless-ngx, but it should really be doing this on its own. If it doesn't, and you're not able to edit the config, you should probably open a bug report.
If you're just using this to send mail to yourself, though, you can work around it by sending direct to Gmail with unauthenticated SMTP. I have this configured on a couple systems. You might need to hit "not spam" on the messages a couple times because they'll probably get flagged at first.
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