How can I specify a login for sending SMTP with SmtpMail class?
It's done through the SMTP mail server. If you are using Microsoft SMTP
mail server, you can perform the administrative settings in the property
pages of the Internet Service Manager console.
You don't; at least not with the SmtpMail class or anything in the
System.Web.Mail namespace. You can write admin task oriented script for the
Internet Service Manager (ISM) console. Sample scripts are contained in a
subdirectory of the inetpub directory.
SMTP is a protocol and it does not address security concerns like
authentication & authorization. The SmtpMail object is a wrapper to be used
by any mailing system/server that follows that protocol. It's neither a bug
nor a problem specific to Microsoft.