I might be wrong but I think it’s a function of Discord that can’t be turned off.
Last mention I can find of it:
Automated e-mails
Also, just as a quick update as we have had lengthy discussions about e-mail newsletters and privacy issues before: The default option across Discourse forums afaik has been for the mailing list mode to be enabled for each user, which has meant that each user has had to individually and manually find the correct opt-out setting in their site preferences. This has now changed, so that the default is that mailing list mode is disabled . Meaning the automated digest emails will go out to even fewer users than before.