Possibly missing some TXT /CNAME records (for SPF , DKIM, DMARC if so?)
You are using a hostname like yourdomain.com which is (proxied) rather than the unproxied (DNS-only) like mail.yourdomain.com (possible this one not existing or is instead of being ) at the DNS tab of Cloudflare dashboard, in your e-mail client for sending/receiving server (MS Outlook, Mozilla Thunderbird, etc.)
May I suggest checking below article if your e-mail records (usually the A mail and the MX record) are configured properly while you are using Cloudflare for your domain name:
My provider has checked everything and assured me that my DNS configuration is ok…
I wrote to a server that rejected my emails last week and they wrote me that the fault is Cloudflare’s and not mine
They told me to write here and ask for a fix, as my IP is shared with a phishing site
They whitelisted me temporarily.
Same thing with Proofpoint Cloudmark, I had to wrote asking to whitelist me
but today I had other mails rejected
This is driving me crazy
Can you please check this, blacklisted, which apparently is sharing the IP with me?
Kindly, make sure this A mail hostname at the DNS tab of Cloudflare dashboard it set to unproxied (DNS-only).
That’s why you’re experiencing issues.
Nevertheless, if you’re sending e-mails via contact form or some other web app, make sure you’re using correct SMTP outgoing server (mail?) with assigned credentials.
Nevertheless, you might want to modify your TXT record related to the SPF, as far as your MX records are using “One” service provider alongside your hosting/server (which is using mail hostname and not MX hostname).
Maybe something you changed or what else could be inspected using the Audit Log at Cloudflare dashboard when you take a look back a while what happened if your A mail hostname or any other changed due to some reason.
Secondly, decide and configure your “web form” either to send out e-mails from your “web form” via:
a) using unproxied (DNS-only) hostname mail.dogwelcome.it
b) or rather using the MX of “One” provider’s steps as for example for MS Outlook as described at the link from below:
Double-check which of your incoming/outgoing hostname (server) at Thunderbird you’re using.
I changed nothing at all - for this reason I don’t understand why all went ok for years and now I have this issue.
My provider told me that everything is ok as always and sent me the configuration I sent you…