Here are some software options you can consider to help prevent employees from accessing your clients' email addresses:

Microsoft Office 365 - Microsoft Office 365 includes access control and data loss prevention (DLP) features that can help you prevent employees from accessing your clients' email addresses.

Symantec Data Loss Prevention - Symantec Data Loss Prevention is a comprehensive software solution that includes access control and DLP features, as well as email encryption and monitoring capabilities.

McAfee Total Protection for DLP - McAfee Total Protection for DLP is a software suite that includes access control, DLP, and email encryption features, as well as email and employee monitoring capabilities.

Forcepoint DLP - Forcepoint DLP is a software solution that includes access control and DLP features, as well as email and employee monitoring capabilities.

Proofpoint Email Protection - Proofpoint Email Protection is a software suite that includes email encryption, access control, and DLP features, as well as email monitoring capabilities.