What is the correct PCAP filter to capture all TCP traffic going to or from host 192.168.0.125 on port 25?

tcp.src == 25 is not a valid Wireshark display filter field (the correct field is tcp.srcport), and it would only match traffic originating from port 25, missing traffic destined for port 25.

The syntax host 192.168.0.125:25 is not valid in either BPF capture filter or Wireshark display filter syntax; host and port must be declared as separate filter primitives joined by a logical operator.

port 25 and host 192.168.0.125 is valid BPF/libpcap capture filter syntax used by tcpdump, not Wireshark display filter syntax, so it cannot be entered in the Wireshark display filter bar.

In Wireshark display filter syntax, tcp.port == 25 matches TCP segments where either the source or destination port equals 25, and ip.host == 192.168.0.125 matches packets where that address appears as either source or destination. Combining both with 'and' correctly captures all TCP port 25 traffic flowing to or from the specified host.