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 (email@example.com: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.
DMARC Policy Issue
Each website domain host must have up-to-date DMARC in order for email messages to be accepted by email service providers. If your company is receiving failure messages like the ones below (note the reference to DMARC policy) you will need to contact your email domain provider and ask them to reach out to the service providers and provide updated DMARC details to each email provider.
Here is additional information about this issue from Google Answers: https://support.google.com/mail/answer/2451690