You may run into this error due to two reasons.
1. Invalid Authentication Type/Port
Many SMTP clients reject Port 465 (SSL) requests and expect a TLS connection using Port 587. Therefore, the recipient server may deny your request if you are not using a TLS connection.
2. Too Many Bounced Emails
If you have sent multiple emails to several email addresses that do not exist, the SMTP server may limit your requests or even block email sending for a brief period.
There is no proper way to solve the error. However, you can try the following solutions to see which one works for you.
1. Verify SMTP Settings
Ensure that you use the proper port and authentication type required by the recipient SMTP server. Using TLS instead of SSL is recommended as most email clients reject connections on Port 465 as this is deprecated.
2. Verify Email Validity
Review the recipients of your email and ensure that each email address exists. If there is suspicion of an invalid email, take it out and send the remaining emails. Too many bounces can harm your sender's reputation, which can cause your provider to set rates for your emails.
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