The reason for this is that when you are not connected at home you are trying to send email through the outgoing mail server at your ISP.  When not at home, this mail has to be forwarded from a different ISP to your outgoing mail server.   Because it would be easy for spammers to use this method to send spam, most ISPs do not allow forwarding to another ISP's email server.  There is nothing you can do about this.  On the other hand, most ISPs have web mail service.   If you log in to your account through web mail, you can send email from there because you logged into you ISPs server.   I have this problem all the time and I hate the awkwardness of webmail.  So, when I am away from a connection to the internet through my ISP, I use Outlook or Outlook Express to read me emal and delete the spam and other junk and then decide which emails I need to respond to.  I then log onto my ISPs webmail and respond to those emails that need a response from there.

 

Steve

Steve, thank you so much!