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.
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 sizesup to 25 GB worth of attachments 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.