A method often used by spammers. They send various emails or passwords to a domain hoping that one will succeed.
The server detects a dictionary attack when a user attempts to send email to four consecutive invalid email addresses. When this happens the server believes it is getting spammed by someone who is trying a bunch of addresses to see if any of them get through.
The dictionary attack block will be removed after 1 hour if there are no more than 4 erroneous emails in a row.
You must stop sending emails and pause your mailing lists. Then wait approximately 1 hour, and it should clear itself.
If you were using Outlook, during the wait you can try using webmail to send messages.