First, you'll want to pull an email log for the last couple days. See also: How To - Pull an Email Log
Download the log and open in Excel. Note the 'Error Messages' column. If you see a large number of the following errors:
- 550 permanent failure
- mailbox not found
- email account does not exist
This indicates there is an issue with your email domain.
Example of Email Domain Blocked
'550 permanent failure for one or more recipients (firstname.lastname@example.org:blocked)'
Occasionally, your email domain provider will make updates. Those updates can change deliverability for ALL emails from Salesforce, preventing them from reaching your employees, or anyone with a 'abcroofing.com' email address.
We recommend reaching out ASAP to your hosting provider to review the Email Log errors and work with them to immediately unblock messages originating from Salesforce.