Salesforce log files allow you to figure out what's going on with email deliverability issues.  Here's how it looks:

Search in setup for "email log files":

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​Once there, request a log file to be generated:

That's it!  You will get an email notice when it's done and then you can download that CSV file.  The file will tell you if the email was queued for delivery on one row, then a second row when the email was either delivered or ithe error if it wasn't successful.

Official documentation:
​https://help.salesforce.com/articleView?id=email_logs_edit.htm&type=5
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