Glad it's working for you.
Your ISP's DNS server is slow to respond. DNS is like the phone book for the internet, your computer asks it where to find smartbro.net, then makes that connection. Probably, Sylpheed does not try enough pings, and/or doesn't wait long enough to get an answer, so it gives up. The time it waits before giving up can be adjusted in the preferences.
Google offers free DNS services on 184.108.40.206 and 220.127.116.11, and so does OpenDNS at 18.104.22.168 and 22.214.171.124. I have found these servers to be better than what my ISP delivers, and use them all the time. You can configure your network manager to do this.
Another "trick" is to check for incoming mail right before sending, as this can create a connection to the server that stays alive long enough to send.