I’ve been troubleshooting email send/receive issues for as long as I remember, usually in most varieties of Windows mail clients, including Outlook Express and Windows Mail. Eliminating any remote mail server issues, sending and receiving emails tend to be the most frequent problems encountered–often fixed by diagnosing the network connection (checking settings or rebooting a router), turning off any aggressive firewalls and antivirus programs, or even recreating the user’s mail profile in the client.Again, as long as the issue isn’t related to mail relaying and port 25 blocking (which many ISPs are in the habit of doing), then try the following steps to clear/reset TCP/IP on a client machine:

ipconfig /flushdns
ipconfig /renew
netsh winsock reset

The above requires Administrator privileges to run from a command line. It will also request a PC reboot. The following article also provides more instruction on resetting TCP/IP on a Windows-based machine.