SMTP error 531 indicates that the recipient's email server is over quota or storage limitations and cannot accept your message (phpmailer, jenkins). This means the mailbox is full and has reached its capacity.

What Triggers This Error?

This error solely occurs when the recipient's mailbox has exceeded its allocated storage space. This can happen due to a large number of emails, attachments, or inactivity (emails not being deleted).

Resolving SMTP Error 531 - Solution

Unfortunately, there's no immediate action you, the sender, can take to resolve this error. The solution lies with the recipient:

1. Contact Recipient: Inform the recipient about the error message and suggest they free up storage space in their mailbox by deleting old emails or increasing their storage quota if possible.
2. Retry Delivery Later: Once the recipient has addressed the storage issue, you can retry sending the email at a later time.

Remember, this error is temporary and resolves itself once the recipient frees up space in their mailbox.