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Outlook may become sluggish or unresponsive at the moment large files are attached to an email when using Outlook’s Attach File button

Karla Marsh
Mail Express - DEPRECATED


  • Mail Express, v3


Outlook may become sluggish or unresponsive at the moment large files are attached to an email when using Outlook’s Attach File button. This can also occur shortly after sending an email with large attachments that have been attached with Outlook’s Attach File button. The delay may become more pronounced the larger the size of the attachments.


The attachment is being managed by the Exchange Server instead of Mail Express and the Exchange Server is limiting the size of attachments.


Outlook does not handle large attachments very efficiently. Attaching large files with Outlook's Attach File button will cause a lag the moment they are attached to an email. This lag is unrelated to the Mail Express Add-in. For better performance, use the Mail Express Attach File button to attach files instead. If the Mail Express Attach File button is hidden, it can be enabled by clicking View > Toolbars > Mail Express in Outlook, as described below.


    By default, Microsoft Exchange allows up to 10 MB of attachments (total) per email. Outlook will prevent users from exceeding this limit and will display the message "The attachment size exceeds the allowable limit" if they try to do so. As a result of having this limit in place and set to 10 MB or less, users will be less likely to experience this delay as they can only attach larger files using the Mail Express Attach File button. If it is desired that the Add-In is run in transparent mode, meaning that the Add-In’s primary toolbar all other user interface elements of the Add-In are hidden, then users will not have access to the Mail Express Attach File button. This means that the Exchange message size limits will need to be increased to allow users to attach files larger than 10 MB with Outlook’s Attach File button and when users do so, they may experience this delay. This is essentially a tradeoff between running the Add-In transparently and having to deal with Outlook’s performance issues with how it naturally handles large attachments versus running the Add-In in visible mode. Allowing users to attach files with the Add-In's Attach File button provides a better user experience with attaching large files.

    To hide or show the Mail Express toolbar

    • Right-click a blank area of the Outlook toolbar/ribbon, then click Mail Express.
    • -or-

    • On the Outlook main menu, click View > Toolbars > Mail Express.

    Email Message Toolbar

    When you create a new email, the Message ribbon/toolbar allows you to specify the file(s) to attach and configure Send Options that are specific to that email.

    Last Modified: Last Year
    Last Modified By: kmarsh
    Type: FIX
    Rated 1 star based on 9 votes.
    Article has been viewed 22K times.
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