From mail2web.com Wiki
What is the "clean up attachments" link for?
The "clean up attachments" option in mail2web.com does not clear your attachments in your POP3 mailbox and does not delete the attachment itself in the email. It merely deletes the attachments on our server. When we retrieve email which has an attachment, mail2web.com downloads the attachment to our server. By clicking "clean up attachments", it will clear the attachment from our server. It is a partial extra security option. When you are viewing/downloading attachments from mail2web.com, it is temporarily stored on the server, so you can download it. These files are deleted every hour, but if you wish to be extra cautious, you can delete the attachment immediately.
If you open the email message again after doing this, the attachment will again be copied onto our servers.