Explanation

Users in Dept1 should use S/MIME encryption . S/MIME is a standard for public key encryption and signing of email messages1. It allows users to digitally sign and encrypt email messages as required using certificates1. S/MIME provides end-to-end encryption and ensures message integrity and sender authentication1.
Email messages sent from Dept2 to an external email domain should use TLS encryption1. TLS is a protocol that encrypts the connection or session between two servers1. It protects email messages during transit only and does not require any action from the sender or recipient1. TLS provides opportunistic encryption and ensures message confidentiality and server authentication1.
Email messages sent from Dept3 to an external email domain should use Microsoft Purview Message Encryption2. Microsoft Purview Message Encryption is an online service that encrypts email messages and attachments using rights management policies2. It allows users to send encrypted email messages to anyone inside or outside the organization using various email services2. Microsoft Purview Message Encryption provides persistent encryption and ensures message confidentiality and authorization policies2.