The email sender may have a low spam score. What should you do to troubleshoot it?
Do other people get the sender's emails? If so, has it ever worked? If not, check the destination email address. Can the destination email address send an email to the first sender with the problem? You could then reply. Maybe you accidentally typed in the wrong email address.
If you think a "Domain Name System Block List" is affecting you, see this: https://www.spamhaus.org/faq/section/DNSBL%20Usage
To address the spam score issue (which may take days), see these links: