This is exactly what I did to fix my problem:
To use OpenDNS, follow these steps:
1. Open the Network pane of System Preferences.
2. In the list on the left, select the interface that’s currently providing
your Internet access (usually AirPort or Built-in Ethernet).
3. In the DNS Server field, fill in the following:
208.67.222.222, 208.67.220.220
4. Click Apply, and then close System Preferences.
5. To verify that the new DNS servers are working, visit any Web site
in your favorite browser.
If DNS delays were causing the sending problem, sending mail should
go much faster now.
Just try but watch the settings as they were so you can always change back.
clogz14