Outlook 2010 has a new feature that prevents you from sending messages larger than your SMTP server accepts. If you have an Exchange mailbox, Outlook gets the message limit from the server. Because other servers don't share this information with Outlook, it sets a default limit of 20 MB. If your SMTP server supports larger messages (or smaller), you can edit the registry to prevent the warning. For the relevant registry settings, see Outlook 2010's Default Attachment Size