email not received by sender

I've had this problem many times.  First you need to check any filter you have in place for incoming email (check for that message) . If it is received shown as delivered successfully then check the junk mail folder in outlook if this is the software used by the client.  If the incoming email doesn't get received to your filter, you can label that sender in a whitelist group to bypass any filter rules that they could be blocked by. Have them send another message, if its still not being received, check with the person sending to your email server, check to see if they receive any bounce back message or any error of failure to deliver.  If not, then the email issue would be on the senders end, it could be getting held up in there email filter.  These are the first steps I would take.

Powershell Command to whitelist a domain
set-contentfilterconfig -BypassedSenderDomains domain.com
White list User
set-contentfilterconfig -BypassedSenders JohnDoe@domain.com

Global Accept List: Add IP addresses of sending hosts in Connection Filtering's Global Accept list (Global Settings | Message Delivery -> properties | Connection Filtering tab), and enable Connection Filtering on SMTP virtual server properties (General tab | Advanced button next to IP address | select IP address -> Edit | check Apply Connection Filter). This allows messages from the sending host to bypass Connection Filtering - inbound messages will be accepted even if the sending host is listed on a RBL - and IMF as well.