Mail sent from Email Security.cloud infrastructure are being greylisted by the recipient's infrastructure forcing some mails to retry.
451 4.7.1 Greylisting in action, please come back later
Greylisting ensures that mail is legitimate by asking the sender to try again later. Legitimate senders will do so (such as our mail systems) and generally spam senders will not. When greylisting occurs the connection is temporarily failed with a 400 series SMTP error.
Greylisting can happen at different phases during the SMTP session:
SMTP HELO/EHLO Greylisting
SMTP MAIL Greylisting
SMTP RCPT Greylisting
SMTP DATA Greylisting
To ensure greylisting is not applied on SMTP connections from Symantec.cloud delivery towers, the recipient administrator would need to clear the Symantec.cloud IP Ranges reputation on their greylisting systems.