May be Wireshark not capturing total network ?

Dear Geeks,

I am in process to find out the spamming computer on the network.

So I have captured network traffic by using Wireshark 1.10.8 on Windows 8 and N/W Card is Wi-fi ( Qualcomm Atheros QCA9565 ).

I have sent an email while capturing the network traffic by Wireshark for test. When I filter the captured traffic by using the filter "tcp.port == 587" ,it showing only the mail sent by my (sent by me ) laptop (same computer on which Wireshark is running). Here port no. is 587 because the Outlook is configured with 587 port for SMTP for all computers in the network.

But in the capture I am seeing the different IPs and computer names which are on the network (not only mine),it seems it is capturing the packets in network but not sure what is happening in case of mail sent filter ( for sure that someone on the network have sent the mail while the network traffic captured ).

Please suggest me to resolve the issue.