Unfortunately, Netscape 4.7 and similar old versions require you to enter your password every time you send email. You can upgrade to Mozilla Thunderbird if you'd prefer.
Netscape 4.7
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Go to the Edit menu and click Preferences
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Click the "+" in front of "Mail & Newsgroup Settings" unless it's already a "-"
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Click the Mail Servers item
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On the right, in Outgoing mail server username, put your username (usually the first part of your email address)
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Click OK