Apple mail sent email to weird address

I sent an email to someone via my Apple Mail app. I thought it sent correctly, but the person I emailed told me they never received the email. So I went to the Sent mail to confirm.

I looked at 'Sent' mail and in the view window it looks like it was sent to the correct email. But when I tapped it open to see the details, the email was sent to 2 people. The original (I'm assuming) and a new one with the weird ending: g8e3j8a17641xq_7bdx1559@icloud.com.

I did a malware scan on my computer and there is nothing detected. Is this just a weird bug or something to be concerned about? I didn't find a matching email in my contacts.

thoughts?