Did you tried to add the username in the URL(screen below), and the password in Attachment password property in the email account? Also you must ensure that there is sufficient free local disk space in the Snaplex, since Email Sender Snap first downloads SFTP or SMB files to a temporary location in the local disk before sending them as attachments.
Yes and yes. For SMB, per the example in the screenshot below, we did add the user name in the URL and added the password in the Attachment password property field. There is plenty of space on the local disk.