This error can occur for various reasons, depending on the SMTP server you use.
In most cases, you may run into the error if there is an issue with the recipient's email server. In other cases, you may run into this error if the outgoing SMTP port is blocked from your end. However, if you use an SMTP client such as AuthSMTP, you will run into this error if you exceed your monthly message or data quota.
There is no proper way to fix this error, but you can try the following to see which one works for you.
1. Contact Server Administrators of Recipient SMTP Server
You can contact the server administrators of the recipient SMTP server and gain additional assistance from their end. It could be that you are likely using the wrong SMTP Port or the wrong authentication type, so verifying this information from them may help solve your problem.
2. Contact your network administrator
On some occasions, your outgoing SMTP port may be blocked by your network administrators. Therefore, communicating with them and having it open and unblocked may solve this error.
3. Upgrade Monthly Quota
For users using SMTP servers with a monthly quota (AuthSMTP, SendGrid, MailChimp), you can either upgrade the monthly ration or wait until the quota resets.
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