I had a similar problem and there is a technote called E-mail is re-sent by user who does not have delegate rights to original sender’s mail file. This was the answer This issue can happen if User B has added the “Send” button to one of his/her toolbars.
This button is usually located on the Universal Toolbar (the one with Save, Open, Print, etc).
If a user clicks the toobar Send button, the email they have selected or opened in their mail file will automatically (without any prompts) re-send the email to everyone on the recipients list. It will also show as “From” the original sender and “Sent By” the person who just re-sent it to everyone. This does not require any access to the original sender’s mail file, as Notes is actually sending it from the “Sent By” user’s mail file.