This is spam protection being triggered because, usually on a group send, one or more mailboxes are inactive or the address is incorrect.
Mailboxes do get deleted and people switch to a different mail service provider. And it's not unknown for addresses to be entered incorrectly.
You could check at mailtester.com to see if that indicates a problem.
453 4.1.1 Recipient Invalid (TT503) (in reply to RCPT TO command) Implies the mailbox is temporarily unavailable.
What's the email domain of the recipient?
The mailbox may be full, server not responding, network issues.
Don't change anything just try again later is the usual tactic if everything else is ok.