I use Outlook at work, and sometimes people send big attachments without zipping them up. Often they’re documents which are in the document management system anyway. Sigh.
Anyway, it’s easy enough to compress them within Outlook.
1. Open mail item.
2. Save attachment(s) to a temporary directory.
3. Zip (or whatever) the file. (See? 2Mb XLS down to 300Kb. Why didn’t the sender do that?)
4. In the email, click Edit / Edit Message on the menu. (This feature is a boon for fraudulent modification of emails, but also for compressing attachments.)
5. Right click on the attachment and Remove.
6. Drag the zip file into the email. (For some reason you can’t use the menu to insert an attachment like you can when composing. And the drag-drop has to land in the body section of the email.)
7. Save and Close the email.
Voila, a bunch of space saved.
I haven’t explored to see if the same method can be used in other mail clients.