Hello.
this can be due to their email didn’t pass the SPF, DKIM and DMARC checks. These are standard email domain verification DNS records that help determine if the email is really from the legitimate sender and is it spoofed to look like the email address you trust. If the sender doesn’t configure these correctly their emails can be marked as spam .
I’ll suggest that you use this instruction here to view the email header. And look for mentioning of SPF, DKIM and DMARC and see if they failed.