Not enough space on the disk.
- Number of recipients the email message has – receiving server cannot parse which of the recipients it is supposed to deliver to.
- Out of memory condition due to a file overload.
- May depend on too many messages send to a particular domain.
Step 1: Try Again By Sending Smaller Sets
Try again by sending smaller sets of emails instead of one big mail-out. For example, separate the big email to smaller one.
Step 2: Recipient Is The Only Receiver
Attempt a new email message to the failed recipient, ensure they are the only recipient for that message.