Browse bounce messages for network connection below
4.4.0
Unknown issue with network connection
Network Connection Problem
4.4.2
Network connection timed out
Network Connection Problem
4.7.11
Encryption needed for network connection
Email Encryption Issue
5.4.0
Unknown issue with network connection
Network Connection Problem
5.4.2
Network connection timed out
Network Connection Problem
5.7.11
Encryption needed for network connection
Sender Account Issue