An administrator is unable to receive e-mail. While troubleshooting the problem, the administrator is able to ping the local mail server IP address successfully from a remote network and can successfully resolve the mail server name to an IP address via the use of the nslookup command. At what OSI layer is the problem most likely to be found?

The administrator can ping the mail server (Layers 1-3 working) and resolve the mail server name via DNS (Layer 3 + DNS application working), but email retrieval fails. Since lower-layer connectivity is confirmed and name resolution works, the problem must be at the Application Layer where email protocols (SMTP, POP3, IMAP) operate. This could involve email service configuration, authentication issues, firewall blocking email ports, or the email daemon not running. Physical, data link, and network layer issues would typically prevent ping or DNS from working.