We are attempting to deliver emails to several organizations that are deferring our attempts. Error/defer message is "450 4.3.2 please try later".
These are email newsletters that are opt-in.
I checked the sending IP here http://ipremoval.sms.symantec.com/lookup/ and the IP does not have a bad reputation.
When receiving the 450 message we retry in 60 minutes.
I've seen some indication that this error is returned when an attempted directory harvest attack is suspected. However - we have very clean lists...
Since there are multiple organizations throwing this same error - and we've identified the Symantec Messaging Gateway as the origin - we must be on some type of block list.
I could not find an appropriate support form or email address for deliverability issues.
Any insight or thoughts or suggestions from anybody?
Thanks