Hi Sherman,
Phaneendra covered the outbound scenario well. One additional consideration is that transport encryption and attachment-level encryption are separate requirements.
The Custom SMTP Server integration supports outbound email using TLS 1.2 or 1.3 through STARTTLS or SMTPS. However, I could not find official documentation confirming native file-level attachment encryption or bidirectional encryption and decryption for threaded inbound and outbound emails.
For an external gateway solution, it would also be important to preserve the In-Reply-To and References headers, since Genesys Cloud uses them to maintain the email thread.
It may help to clarify whether the requirement is S/MIME or PGP message encryption, password-protected attachments, or another type of file-level encryption, since each would require a different architecture.
References:
https://help.genesys.cloud/faqs/how-do-i-ensure-my-emails-are-encrypted/
https://help.genesys.cloud/articles/configure-and-activate-the-custom-smtp-server-integration/
https://help.genesys.cloud/articles/about-acd-email-routing/
I hope this helps.
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Raphael Poliesi
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