The users in a company are unable to send and receive SMTP e-mails using the BIG-IP Virtual Server. The administrator logs into the BIG IP device to verify if the mail pool member with an IP address o

Port 143 is the IMAP protocol port used for email retrieval, not SMTP email delivery, so this filter would capture the wrong type of mail traffic and miss SMTP sessions entirely.

The curl command shown has invalid syntax for this use case and curl is not a packet capture tool - it cannot monitor live network traffic flows on a BIG-IP device.

The tcpdump command filtered on host 192.168.10.100 and port 25 captures only SMTP traffic to and from that specific pool member, since port 25 is the well-known SMTP submission port defined in RFC 5321. This is the standard method on BIG-IP for verifying pool member traffic at the packet level, allowing the administrator to confirm SMTP sessions are being established and served correctly.

Port 143 is IMAP rather than SMTP, and telnet tests TCP connectivity to a single port at a point in time rather than capturing or continuously monitoring traffic flows.