Outgoing e-mail failing with the following error on your cPanel mail server?
503 Valid RCPT command must precede DATA
This can be caused by various factors, such as permission issues, out of space or inode limit reached.
To investigate this issue, a good start would be having a look at the logs to find out more.
1. Using the shell/CLI :
2. Using the cPanel Web interface :
cPanel > Email > Track Delivery > Delivery Report
In that particular case, the following message was shown :
LMTP error after RCPT TO:<firstname.lastname@example.org>: 552 5.2.2 <email@example.com> Mailbox is full / Blocks limit exceeded / Inode limit exceeded
The last portion of the message is self explanatory, looking at the account quota, it was out of space. Increase the space of the account to fix this issue.