SMTP Error 431
Description: Not enough storage or out of memory.
What does this mean?
The actual message may seem a little misleading. This is not in relation to the receiving/destination server running out of memory or not having enough storage for delivery. This is actually in regards to the amount of recipients the email message has. Suggesting that the receiving server can not parse which of the recipients it is supposed to deliver to.
How to fix SMTP Error 431?
Considering this error is specific to having too many recipients, attempt a new email message to the failed recipient. Make sure they are the only recipient for that message.