At the start of 2022 GMail started requiring SPF on all domains that send to it or it will produce a error:
"Be careful with this message: Gmail could not verify that it actually came from YOUR DOMAIN. Avoid clicking links, downloading attachments, or replying with personal information."
The resolution is to ensure SPF is added to your domain. We have a article guiding this here: My e-mail is going to the bulk or spam folder!