I have referenced this comment from a similar post:
https://www.symantec.com/connect/forums/issue-delisting-ip#comment-12223991
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Hello all,
Please note the https://ipremoval.sms.symantec.com/ site is not managed by the Email Security Cloud team, as it can also include listings by Symantec Messaging Gateway product. Please follow the intructions therein for any de-listings.
If you believe your email has been stopped by email security.cloud error, see the following:
If you have seen your mail stopped with a 553 message filtered error this means we have deemed that mail to potentially be spam and have stopped it as such.
If you believe we have stopped this incorrectly and you are a Symantec Email Security.Cloud client please follow this process in order to address this issue.
https://support.symantec.com/en_US/article.TECH233678.html
If you believe we have stopped this incorrectly but you are not a Symantec client we can still assist you. Please follow the process in this link
https://support.symantec.com/en_US/article.TECH82881.html