Email addresses are added to the Amazon SES Suppression List when any Amazon-based sender gets a Hard-Bounce response when trying to send to that email address. It is not related to emails sent from that address.
You'll need to ensure that the email address is not giving any hard bounce errors - even a single hard bounce, for any reason, can trigger the suppression.
From Removing an Email Address from the Amazon SES Suppression List - Amazon Simple Email Service:
Amazon SES maintains a suppression list (formerly called a blacklist) of recipient email addresses that have caused a hard bounce for any Amazon SES customer within the past 14 days. If you try to send an email through Amazon SES to an address that is on the suppression list, the call to Amazon SES succeeds, but Amazon SES treats the email as a hard bounce instead of attempting to send it. Like any hard bounce, suppression list bounces count towards your sending quota and your bounce rate.
The only way you will know if an address is on the suppression list is that you will receive a suppression list bounce when you send to it. There is no way to query the suppression list in advance.
Important
As with any email address that hard bounces, you should remove addresses that cause a suppression list bounce from your mailing list unless you are absolutely sure the address is valid, because suppression list bounces count towards your bounce rate and a high bounce rate puts your account at risk of being shut down. If you remove an address from the suppression list when it is indeed undeliverable, then the next time you or another Amazon SES customer sends an email to that address, it will hard bounce and the address will go back on the suppression list.
If you are sure that an address on the suppression list is valid, you can remove it from the list by using the following procedure. Although each AWS region has a separate suppression list, if you remove an address from the suppression list of one region, the address is removed from the suppression list of all regions.