Why client's email addresses are constantly added to the bounce list?

I have a lot of clients using JotForm (both paid and not paid).

MANY, MANY, MANY of my clients keep on getting their emails on the 'blocked' list and they stop receiving online submissions from their website. This is very bad.... obviously. I unlock them and they'll receive a few emails, and then it happens again.

Are you aware of this? I've been using your service for 5+ years and I've noticed this just started happening over the past 6 months. I have spent a lot of time fiddling with settings for clients to resolve this to no avail.... do you all have a solution? Why is this happening?

My contact info:

Zak Kates

zakkates@simple-edge.com

Apologies for the inconveniences this may have caused to you. 

An email address is added to our bounce list due to several reasons, may be that the destination mail server was not available at the moment the email was sent, the email was blocked due to security filters, or the email was marked as spam by the email provider. 

I would suggest you to check this guide in order to avoid bouncing issues: https://www.jotform.com/help/208-How-to-Prevent-Email-Bouncing-Related-Issues 

You may also ask them to ask their email providers to whitelist our IP addresses: https://www.jotform.com/help/145-Whitelisting-JotMails-IP-Addresses 

Another way to avoid bouncing issues would be setting up an SMTP sender, this guide will provide you more information about it: https://www.jotform.com/help/244-How-to-Setup-SMTP-for-a-Form 

If you keep experiencing the issue, please let us know.