I'm getting this error when trying to submit a contact form (contact.php)
Messages with multiple addresses in From: header are\n550-5.7.1 not accepted. For more information, go to\n550-5.7.1 https://support.google.com/mail/?p=RfcMessageNonCompliant and review\n550 5.7.1 RFC 5322 specifications. 6a1803df08f44-91f23eb25c3si13820796d6.586 - gsmtp DT=0.445s

Any ideas?

Apparently "Gmail does not support multiple email addresses inside a single "From" field or header"
So use the CC field for more recipients.

But I'm not specifying recipients... or I'm not sure where to specify them or why multiple are being sent to... I'm not exactly sure where the contact form emails are set.

Admin backend > Plugins > contact_form > Contact recipients

There is only one contact specified there, but I guess the error relates to the "from" not the "to" so that field doesn't seem relevant.