The Mail Express Outlook Add-In interface is hidden by default (transparent mode); however, the Add-In can still process emails depending on the rules defined by the administrator in the Mail Express administration interface. When the Add-In’s user interface is enabled, the Add-In is no longer in transparent mode and all of the interface elements of the Add-In are accessible. This means that the Add-In’s Attach File button is available in addition to Outlook’s Attach File button. If a file is attached via the Mail Express Attach File button, the attachment will be managed by Mail Express regardless of the policy settings. It is assumed that since you are using the Mail Express button, you want to use Mail Express to manage the email. Mail Express replaces the attachments with small "surrogate" files that contain the path to the actual file on the file system. This surrogate file has the same name as the file being attached except that it ends in a ".mailexpress" suffix. This method greatly improves performance when attaching files as well as saving and sending emails with large attachments, because the amount of file data attached to emails is reduced. The behavior regarding the Attach File buttons and the surrogate files in Outlook 2003 differ slightly from newer versions of Outlook.
For Outlook 2007 and later
For the most optimal experience, you should leave your mail server and Mail Express message size limits set to their default values (10 MB). When users try to attach files that would exceed the message size limit via Outlook’s native Attach File button, Mail Express will automatically replace the attachments with surrogate files. Even with the message size set low, users can send emails with unlimited file sizes per email with Mail Express.
Keeping the message size limit low ensures that users are unable to send huge files unless they use Mail Express to do so. Preventing users from sending huge files can avoid problems, such as when a user sends a large attachment to everyone in the company, potentially slowing down the mail server and the corporate network as the attachment is delivered to every recipient. Many of the popular email providers like Gmail, Yahoo Mail, and Hotmail have a 25 MB message size limit. Ensuring that the mail server and Mail Express message size is configured within these bounds reduces instances in which emails sent to other email servers are bounced.
You should set the Mail Express message size limit to the same value as mail server Server's message size limit or less. With the message size limit set this way, Mail Express can prevent users from receiving the "Attachment size limit has been exceeded" message when using Outlook’s Attach File button. Note that users may still get this prompt if they drag-n-drop one or more files that would exceed the message size limit as Outlook does not allow add-ins to alter its drag-n-drop behavior.
For Outlook 2003
When using Outlook 2003’s Attach File button, users may be prompted that a particular attachment cannot be attached because it exceeds the allowable limit ("The attachment size exceeds the allowable limit."). This occurs because Outlook is trying to enforce the attachment size limits defined in the mail server. Therefore, it is important to realize that if the Add-In is running in transparent mode, the attachment size thresholds in the mail server will have to be increased to allow users to send larger attachments with Mail Express. Keep in mind that by increasing these mail server limits it may be possible for users to send emails with large attachments through the mail server without the attachments being managed by the Add-In (e.g., if the Add-In is disabled), which could affect the performance of the mail server.
In Outlook 2003, attaching files with the Mail Express Attach File button provides the following benefits over Outlook’s standard Attach File button:
Large emails can be attached more efficiently with Mail Express, because a surrogate file is always small. When attaching large files through Outlook’s standard Attach File button, Outlook may appear to hang while it is processing the attachment and writing it to the file system in several places. The Mail Express Attach File button eliminates this delay.
If the mail server administrator has defined attachment size limits, then Outlook will display the message "The attachment size exceeds the allowable limit" when attaching files that exceed these limits using Outlook’s Attach File button. The Mail Express Attach File button does not suffer from this problem.