The From: and ReplyTo: addresses are set by the proper methods on the EmailRequest class. The account (user) and password used to login to the email sending server is only specified in the connector's attributes. Some email servers restrict the From: address to be the same as the account, though usually not the ReplyTo. If the From or ReplyTo is not specified in the EmailRequest, then the connector's Default Sender's or Reply To settings are used. If there is no default From (and no From in the EmailRequest) an error will be posted.
Most email sending servers have tightened their security requirements, so that they allow sending only as the logged in user. You'll have to negotiate with your sending server provider if you want to send as others from a single account. The default email connector can login only as a single account. If you have more than one account you'll need either to write your own email connector or use the JavaMail API (
https://java.net/projects/javamail/pages/Home) from your snap-in directly (the default email connector uses JavaMail).
We have found that restrictions on ReplyTo have not been tightened, and you're more free to put other addresses there.