Welcome to our forum!
Looks like the issue is more related to Office development, please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client. The following is for informational purposes only.
According to my search, if you wish to retrieve the fully qualified email address of the sender, use the SenderEmailAddress property. More information, see:
MailItem.SenderName property (Outlook) | Microsoft Learn.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.