How do we allow an email with password protected attachments securely over an email gateway like Cisco Ironport, Fortimail etc.?

You have couple of options. Explore if you can allow specific users or groups (e.g., finance or legal teams) that require sending or receiving such files and define exceptions for them in the policy.

or, educate trustworthy users (I know it's a bit dichotomous in times when everyone is talking about zero trust architecture) to include an agreed-upon subject line or header that indicates the attachment is expected and password-protected (e.g., "[Secure Attachment]") and create an encryption bypass policy rule if the gateway vendor supports it.

Regularly monitor and audit the usage of encrypted attachments being allowed. Track patterns to identify potential misuse or suspicious activity. Create alerts for an unusual volume of password-protected attachments from a particular user or to specific recipients.

Finally, if vendor support is not available or they are unable to answer your query, try ChatGPT.

NIST offers guidance on securely handling email communications, including password-protected attachments, by recommending encryption methods to protect the confidentiality and integrity of files. For detailed information on securing email systems, you can refer to NIST's guidelines at this link: https://doi.org/10.6028/NIST.SP.800-177r1.