JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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That is because your email address has been added to our bounce list:
firstname.lastname@example.org is IN Remove the bounce list
Reason(s) : smtp; 554 4.4.7 Message expired: unable to deliver in 840 minutes.<452 Domain size limit exceeded>
I've fixed this for you, but we recommend checking this guide to prevent the issue from happening again:
The whitelisting part is important.