Putting the answer from the link in the comment for easier viewing which save clicking steps. Also this is very common problem.
I would run an Email Deliverability Test
Available in: All Editions except Database.com
User Permissions Needed
To test email deliverability:
“Modify All Data”
Salesforce sends email from 54 different IP addresses. If your organization blocks any of these IP addresses, users might not receive all email sent from Salesforce.
To verify your organization can receive email from every Salesforce IP address:
Click Your Name | Setup | Administration Setup | Email Administration | Test Deliverability.
Enter your business email address.
Click Send. Salesforce simultaneously sends a test message from all 54 IP addresses to your business email address. Each test message specifies the IP address from which it was sent.
Check your business email account to make sure it received all 54 test messages. (as of 6 Apr 2021, SFDC only sends 16 test messages)
If you received less than 54 test messages, your organization's email administrator must whitelist the Salesforce IP ranges on your organization's email server.
Whitelisting an IP address allows the email server to receive email from an IP address that might otherwise be blocked. The Salesforce IP ranges are:
184.108.40.206 to 220.127.116.11
18.104.22.168 to 22.214.171.124
126.96.36.199 to 188.8.131.52
184.108.40.206 to 220.127.116.11
18.104.22.168 to 22.214.171.124
126.96.36.199 to 188.8.131.52
If your organization activates email relaying, your email administrator only needs to whitelist the IP addresses Salesforce uses for email relaying (184.108.40.206, 220.127.116.11, 18.104.22.168, 22.214.171.124, 126.96.36.199, and 188.8.131.52).
For information on email relaying, see Setting Up Email Relaying.