UBB.threads PHP Forum Software Community Forums General Suggestions and Feedback standard emails from UBBT treated as SPAM

For your host to do something to your DKIM certs, they would have had to edit your forum phpmailer script, or with UBB.threads 7.7.0 (and newer), log in to your forum Control Panel and edit the settings. Does that sound like something your host would do?

I just had your forum send me an email.
DKIM failed validation.

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Subject: Emailed post(s) from BritBike Forum

SPF: PASS with IP 23.83.213.43
https://support.google.com/a/answer/33786?hl=en

DKIM: 'FAIL' with domain britbike.com
https://support.google.com/a/answer/174124?hl=en