Nagios Support Forum

I believe Admin -> Manage Email Settings -> "Send Mail From" is what you want.

Log in as nagiosadmin, click on the "nagiosadmin" in the upper-right corner next to "Logged in as", then click on the "Account Information". What is the email that is listed in the "Email Address" field?

Are you using Sendmail or SMTP? (Admin->Manage Email Settings)

You most likely need to change the email address between the "<>" in Admin --> Manage Email Settings --> "Send Mail From" field.
Have you done so already?

Setting up the return address in the "Send Mail From" filed under "Mail Settings" *should* work with SMTP. I am not sure if you can try using SMTP instead of sendmail in your environment to verify that. Another option would be to configure sendmail to rewrite the "from address".