External Reasons for Email Issues
After we have tested the local server and see the local server is working correctly, we can infer that the issue lies with one of the following external problems:

• There exists incorrect DNS information on the remote mail server.
• The remote mail server is malfunctioning or undergoing maintenance.
• The remote mail server may be blocking the sender’s IP.
• Port 25 has been blocked (or intermittently blocked) by the ISP or local network.
• The remote mail server has accepted the message, but the delivery is delayed.
• There is a typo in the email address.
• The IP or domain name is on a blacklist

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