When broadcasting emails, the program engage the SMTP email transaction commands; HELO, MAIL FROM, RCPT TO,  DATA, and QUIT with the mail server to open a connection, send mail and end the transaction.

Your ISP mail server response includes acknowledgment, confirmation and notification of problems with numeric reply error codes such as 501, 502 or 503, etc. See: How Email Works

There are no standard mechanisms for reporting mail system errors except for the limited codes consisting of 3 numbers with text descriptions like;

501 Syntax error in parameters or arguments
Indicating intermittent drop in network line connection that caused your mail client to send erroneous command to the mail server.
Or
502 Command not implemented
Indicating that your ISP mail server did not recognized a command sent. See: SMTP Return Codes Error

Notes:
If the transaction beginning HELO command argument is not acceptable a 501 failure reply must be returned. If the commands in a transaction are out of order a 503 failure reply must be returned.

Command
A request for mail service action sent by the sender SMTP to the receiver SMTP