Assuming you have not added them to your blocked sender list, the most common reason for only specific senders not being able to send to your domain is caused by an SPF check on your mail server.

The bounce message sent back to the sender, might look something like this:

mail.senderdomain.com: john.doe@senderdomain.com 5.0.0 smtp; 550 SPF: 
208.55.48.191 is not allowed to send mail from senderdomain.com 

This bounce occurs when your mail host has SPF checks enabled and the sender has SPF configured on their domain. The way to correct this is to turn off this feature on your mail host.

If turning off the SPF check on your mail server is not possible, another option is to turn on the SPF Delivery Workaround feature in SpamHero ( Settings > Filtering policies > SPF Delivery Workaround).

See also: https://SpamHero.com/support/78970/Resolving_delivery_issues_with_SPF_senders