Regarding To:
be careful not to duplicate To in the additional_headers,
lest gmail already flags it thus:
host gmail-smtp-in.l.google.com [142.251.xx.xx]
SMTP error from remote mail server after end of data:
550-5.7.1 [xxx.xxx.xx.xx] This message is not RFC 5322 compliant, the issue is:
550-5.7.1 duplicate To headers. To reduce the amount of spam sent to Gmail,
550-5.7.1 this message has been blocked. Please review
550 5.7.1 RFC 5322 specifications for more information.