By default elgg uses the php mail() function which typically uses sendmail on the server. Elgg will attempt to set the from address as the address configured in your site settings (Admin -> Basic Settings) but that could be being overridden by the hosting server settings.
The server sendmail isn't all that reliable anyway, would recommend using the HypeNotifications plugin or something to use either a dedicated SMTP account or service like mailgun
Thanks for the info
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