The SMTP 111 error occurs when an issue is present while connecting with the remote SMTP server. For example, you would run into this error due to invalid sender domains or firewall issues.
Additionally, with some mail servers (Amazon SES in particular), you may encounter the 111 error due to invalid login credentials or if the email sending port gets blocked by the recipient.
First, ensure that the email address to which you're sending the email is spelled correctly (especially the domain name). For example, if you're sending the email to a Gmail domain, ensure that the domain is "gmail.com."
Then, ensure that you've provided the correct username and password for SMTP authentication.
Finally, ensure that the email sending port is available for use on your end, and ensure that the port gets accepted on the recipient's end.
However, you can only troubleshoot the SMTP server for firewall issues if the error persists by connecting to the recipient SMTP server through your local computer. If you cannot connect to the server through your local computer, the issue may likely occur on the recipient's end. Therefore you may have to contact the recipient for more information.
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