Had a customer with the same problem last week with FM13 runtime, Windows 10 and Office 2013.
On my own machine with Windows 10, FM13 Adv and Office 2013 and 2016 I have no problem. But that doesn´t help the customer.
One should think, that calling the standard mail program and creating a mail shouldn´t be that difficult.
You can switch to MBS FileMaker Plug-in for sending emails.
Using SMTP via CURL functions. Check SendMail functions to build email with/without HTML, multiple attachments and properly encoded names & texts. Tutorial: Sending emails in FileMaker with MBS Plugin
Or if you like to send via Outlook, please check our new SendMail functions in upcoming 7.2 plugin (currently beta). They can send via Outlook, Thunderbird or other MAPI clients, independent of 32/64bit issues and with a few more options. see Send emails via Outlook
First, verify that FM and Outlook are both the same bit version. If Outlook is 32bit then FM must be 32bit, if Outlook is 64bit then FM must be 64bit.
Second, verify that all updates have been installed for Outlook and Windows.
Third, Reboot the Computer. I listed this after the update because Updates can require a reboot.