Outlook Express 5.5

Why do I need to keep set my email account as Default each time I use dial up access. That’s go : Tools>account, than click on Set As Default.
On LAN it is ok.
There is only one email account which can be used on LAN & dialup.
If I don’t set as default, I can’t send out email.
O/S: Win 95 & Win NT 4