This error predominantly occurs due to an issue in the connectivity from the SMTP client to the SMTP server. This error occurs when the SMTP connection between the SMTP client and SMTP server times out. It is pretty common for port 25 to contain restrictions on the network controls placed by the administrators or ISPs.
The resolution for this error is quite simple; the user must check the network connectivity to ensure that the SMTP client has unrestricted access to the SMTP server over port 25. If the ISP or administrators intentionally deny port 25, the best alternative is to use port 587, which supports SMTP with encryption.
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