When the email list is in the process of refreshing, the list may be locked in the message until the refresh is complete. The recipient list will appear grayed out in the message until the list has finished refreshing at which point you will be able to change the list.

You must wait for the list to complete refreshing before you're able to send the message.

If you cannot select the right type of list for an email message, this indicates the data source may be incorrect for the message. You can change the email data source to select a different list type.