Generally, the SMTP error 459 should provide a detailed error message to deduce the cause. However, few users of the SMTP community found that the most likely reasons for this error are when the recipient email is invalid or if the sender's email sending IP or email is on a deny list.
We cannot identify a proper way to solve it as the error is too generic with only the code. However, you can try these steps to try to solve the error.
1. Verify Email Address
Ensure that the recipient email exists and that you have entered it correctly.
2. Verify IP Blacklist
Ensure that your sender IP is not on a deny list by using MxToolbox. If your IP is on a ban list, contact the Domain Name System-based blocklist service and request them to remove your IP entries from their deny list.
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