Incorrect Handling if SMTP 4.x errors
Your e-mail sending is faulty. You are interpreting the following SMTP response as an UNDELIVERABLE status which is completely false and incorrect, where it is very clear your mailer is not handling it correctly:
[SMTP Status] 451 4.7.1 Please try again later
Most SPAM filters employ damping as a mechanism to inform new senders to just try again later when a new domain is sending to a recipient. SPAMMERS do not resend e-mails.
Adobe is now the first public commercial system that seems unable to understand this. This is VERY poor development practice.
Thanks for reporting this. We’ll look into it.
Adobe Sign Team