Hi Paul,
Based on my test, you can use the following cmdlet export the SMTP addresses to a CSV file.
Get-mailbox -Identity XXX | select EmailAddresses |Export-Csv 'd:\XXX.csv'
After you export the SMTP addresses, you can see the result in the CSV file.
Currently, you can copy the addresses and add the comma ”,” between each two addresses and run the following cmdlet
Set-mailbox –Identity XXX –EmailAddresses “the addresses you edit in the CSV files”
Thanks,
Jason Jiang