Question

Why aren't out-of-office replies appearing in the suspended ticket view?

Answer

Many email providers are moving away from the use of out-of-office replies or they are restricting the domains and organizations to which they will use them to respond. This is in large part because these replies can be dangerous. Hackers can use them in phishing attempts to discover when people are working to improve efficiency on phishing attacks. They also represent a significant threat of creating an email loop. Because not all email systems add the headers that announce the out-of-office response is automated, this creates the potential of an email loop.

For instance, Gmail doesn't send out-of-office replies to Zendesk because they recognize Zendesk as an automated sender of traffic. Many major corporations limit their use of out-of-office replies, or they only send them to known individuals because of the potential threats they represent to security.

As part of responsible sending policies, Zendesk announces itself as an automated system that generates automated traffic and responses. The example headers below help the recipient server to decide what it should and shouldn't do:

Auto-Submitted: auto-generated
X-Auto-Response-Suppress: All
X-Mailer: Zendesk Mailer

For example, you can send out a single email and CC a Zendesk support address to a Gmail user that has out-of-office replies enabled. But you won't likely see their out-of-office response and it may never arrive in your views or your suspended tickets view.  Because you received it in your external email, it seems that the Zendesk account should have also received it. The behavior can also be intermittent. This is because Gmail is updating its responsiveness based only on what they know at any given time. If they don't know that the CC'd address is attached to Zendesk, then they'll send an out-of-office response, as soon as they do recognize that the address is an auto-responder, and they will cease.

This is not an issue that Zendesk can easily solve, or one they can solve entirely on their own. This is because a unique email header meant to be used exclusively for out-of-office replies was never established as part of the RFCs that govern automated email behavior.

For more information, see the article: Causes for ticket suspension.