Relevant Product: Exclaimer Cloud - Signatures for Office 365
You are using Exclaimer Cloud. When a signed or encrypted message is processed, no signature is appended.
This is expected behaviour when using the server-side signature rules - it ensures that security on the message is preserved and un-tampered when the message is delivered to the intended recipient.
It is possible to apply a signature to encrypted messages using client-side signature rules.
This is because the signature is present when the message is composed, so the message is not processed by our servers.
Alternatively, if using server-side signatures, the signature will need to be applied before the message is encrypted. If you use a transport rule to encrypt messages, provided the Exclaimer transport rule has a higher priority and is processed before the encryption transport rule, the message will have a signature applied.