553-Error

In the last couple of months some clients of mine haven't been receiveing messages and when we sent our last newsletter out in Jan, our mail provider said a number of emails soft-bounced due to a symanteccloud 553 error. the sending domain is successatschool.org.

We're not on any spam lists etc and i've submitted an email quite a few days ago with the details, but heard nothing back.

Could someone please look into this?

Thanks