Note I am running Zimbra Desktop 7.1.4 GA (build 11299)

I am having a problem sending emails from two of my three email addresses. I can't send emails from all of the addresses at home, but when I take my laptop to different locations I can only send from one of them. This is strange since the information for all three addresses is almost identical. I have tried changing the default account, but I still have no luck. Does anyone know of a solution for this? Any help is really appreciated - thank you for your time.