A dictionary attack is when hackers floods your mail-server with usernames (email addresses) selected from a dictionary in the form of emails
A dictionary attack on a mail-server can cause excess load and often cause delays in email delivery if not properly configured. Spammers use this trick to gather list of valid email accounts. If your mail-server is configured to send NDR's to for non-existent users, then systems can quickly overload when an attack is seen.
Check your MTA's configuration and make sure it is set to reject emails to unknown recipients and not send bounces(NDR).