About Email Firewalls
Email firewalls are like spam filters, which contain or filter malicious content through a security appliance or server.
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Troubleshooting Test Email Delivery Failures
There are a few reasons you might not receive test campaigns, including spam filters or firewalls.
I know this email address is valid, but it hard bounced
Valid email addresses can hard bounce for many reasons. Reading bounce headers (SMTP replies) can provide insight on what caused an address to hard bounce.
