Gmail has started blocking e-mail that is "not authenticated". This may affect you if you send e-mail from a personal domain name (email@example.com) or a small company or small organization domain name (firstname.lastname@example.org):
"Starting November 2022, new senders who send email to personal Gmail accounts must set up either SPF or DKIM. Google performs random checks on new sender messages to personal Gmail accounts to verify they’re authenticated. Messages without at least one of these authentication methods will be rejected or marked as spam."
What this means to me at the moment is that I can't send e-mail to any Gmail address, until I perform arcane rituals to authenticate my address. (Yes, I am the last person on the Internet to hear about this. Don't make fun of me.)
Technical info in this link:https://support.google.com/mail/answer/81126#authentication