SMTP
502 Command Not Implemented
The command is recognized by the SMTP specification but is not implemented on this server. The server understands the command but has chosen not to support it.
Quando você vê este código
When using optional SMTP commands like VRFY, EXPN, or TURN that the server administrator has disabled, often for security or anti-spam reasons.
Como corrigir
Use an alternative approach — for example, if VRFY is disabled, send a test email instead of verifying the address. Check the server's EHLO response for supported extensions.