Blacklist Detection for ZOHO IP: 74.201.84.163

We were trying to email a client today and were getting this bounce back:

email@email [removed for privacy] INVALID_ADDRESS, ERROR_CODE :554, ERROR_CODE :5.7.1 Service unavailable; Client host [74.201.84.163] blocked using dnsbl.sorbs.net; Currently Sending Spam See: http://www.sorbs.net/lookup.shtml?74.201.84.163

Doing a reverse lookup on MX Toolbox I can see that 74.201.84.163 resolves to:
sender163-mail.zoho.com

And running a blacklist check on 74.201.84.163 shows it's listed on SORBS