The subject line for any emailed tag action must include relevant codes in order to allow the instruction to be processed. In the case of standard email ed responses to Flow360 notifications triggered from the Reply button in an email client these would normally look like this: "(H-ID:12345-C6487-T80736)" - the H-ID code identifies the specific helpdesk issue ID, the C code identifies the sender's contact ID and the T code identifies the specific messaging thread that the message is a response to.
In the case of sending these from an external system the C and T codes may not be available and in these situations the subject line should contain this code string instead: "(H-ID:12345-CE-TE)" - the H-ID code is still required and the CE and TE identifiers confirm that the sender's contact ID and the relevant messaging thread ID are not known. When the email is processed, additional checks come into play to confirm the validity of the sender before the action is processed.
If the checks are all passed then the instruction is processed in the normal way for email responses that contain tag actions and the usual updates and notifications are triggered.