If an email was blocked by a spam filter on the recipient's end, it could cause it to come back as a hard bounce. This spam filter could be in the company firewall, the ISP/data center, or the subscriber's own computer.
A valid email address that has been marked as a "hard bounce" can be removed from your contact list and re-added to erase the history of bouncing. If an address comes back as a hard bounce two consecutive times, it will be marked as a "confirmed bounce" and future campaigns will not be sent to it.
Check the Content of Your Campaign.
Are there things in it that would appear to be spam? There are ways to prevent your email from being filtered
as spam. Be sure to utilize the spam checker
prior to sending out the campaign. This will help increase the deliverability of your emails.
Did the Email End up in the "Junk" Folder?
Many times an email can wind up in your recipient's junk folder if it has been blocked by a spam filter. The more this happens, the higher your odds are of being blocked with future campaigns. To avoid this altogether, request that your subscribers add you to their "safe" email list. This will help to prevent being blocked by filters later on and also lower your bounce rate.
Major ISP Blocking Email
But if you happen to see a major ISP blocking Benchmark in your SMTP headers, it helps if you contact us
immediately. We can then contact that ISP and work things out with them directly.
Learn more about the management of bounces here