Adobe Sign Not Notifying

When recipients sign and send the completed document back, I do not receive an email notification. I have checked with fellow co-workers and the same issue is occurring with them. 

1. We receive some notifications but not all notifications.
2. No rules setup in Outlook, Spam folders have no emails.
3. No changes have been made by I.T to block Adobe emails, hence why we receive some email but not all.
4. We use Adobe Sign online so there are no updates to perform.
5. Email notifications are enabled in Adobe Sign.

Are there any fixes or steps we can take to resolve this issue?