You can increase the maximum size of an attachment or attachments that can be sent in an Outlook email message by changing a setting in Registry Editor. By default, Outlook will not allow you to send an email greater than 20 MB.

In the menu under HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences

With this folder selected, go to Edit . . . New . . . DWORD (32-bit) Value and name the new file as 'MaximumAttachmentSize'.

Double-click this new field, so you can edit it. With decimal selected, enter a limit in KB - so 100,000 for 100 MB.