When saving emails and attachments using MacroView Mail, the Properties pane allows you to edit the default filename of the email and its attachments before they are saved.

For attachments, this can be useful if a different file with the same name as the attachment already exists in the selected location, however for emails, the default file name format used by MacroView Mail ensures each email has a unique name and prevents duplicates of that email being saved in the same location, so you should be careful if changing this.