- You have mistyped your email address (i.e. your email was sent nowhere, or elsewhere)
- Your email was received but misclassified as spam (i.e. your email landed in your spam / junk mail folder, and not your inbox folder)
- Your email address can't receive email (i.e. you've exceeded your quota, your spam filter doesn't allow *.psm file attachments, your spam filter requires a human to answer a challenge, your MX domain isn't configured, etc)




Memoirs of an Email Server:
5.4.1 Recipient address rejected: Access denied. AS(201806281) [*.protection.outlook.com]
Well, your specific problem was a 5.1.2 error. Which means you should: Check the "protonamail.com" part of "…@protonamail.com" for misspellings or missing letters.
Well, your specific problem was a 5.1.2 error. Which means you should: Check the "prontonmail.com" part of "…@prontonmail.com" for misspellings or missing letters.
No MX for yahoo.om
No MX for gmail.coom
No MX for gmal.com
No MX for gmaill.com
No MX for live.con
No MX for gmail.net
No MX for gomail.com
No MX for yshoo.com
No MX for gmail.om
No MX for gnail.com
No MX for hotmail.cocm
No MX for hotmail.xom
No MX for outlook.con
No MX for optonline.ent
5.1.1 Recipient address rejected: User unknown in local recipient table
5.7.1 Message contains spam or virus or sender is blocked
5.7.1 <…@gail.com>: Recipient address rejected: this address does not exist
5.1.1 The email account that you tried to reach does not exist. Please try 5.1.1 double-checking the recipient's email address for typos or 5.1.1 unnecessary spaces. Learn more at 5.1.1 https://support.google.com/mail/?p=NoSuchUser
550 5.4.1 Recipient address rejected: Access denied [prod.protection.outlook.com]
550 5.1.1 Recipient address rejected: User does not exist.
Recipient address rejected: No need for spam. Try again later!
Not delivering to previously bounced address
Server response: 550 5.7.1 The user or domain that you are sending to (or from) has a policy that 5.7.1 prohibited the mail that you sent. Please contact your domain 5.7.1 administrator for further details. For more information, please visit 5.7.1 https://support.google.com/a/answer/172179
452 4.2.2 The email account that you tried to reach is over quota. Please direct 4.2.2 the recipient to 4.2.2 https://support.google.com/mail/?p=OverQuotaTemp