There are many reports here of trouble between Outlook and FMP.
As I Recall, the two main fixes suggested are:
1) Don't use Outlook, use SMTP.
2) Make sure to use 32 bit FMP with 32 bit Outlook and 64 bit FMP with 64 bit Outlook.
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
We can even configure it for Outlook 365. No problem there.
Or if you like to send via Outlook, please check our new SendMail functions in 7.2 version of our plugin. 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
That dialogue is the same one that shows up when 64 bit FileMaker is installed and you have 32 bit office installed (or the other way around). To fix it uninstall FileMaker and reinstall the correct version.
It is probably intermittent because it takes a few seconds for that error to show up, if you are automatically sending emails then it may be the times when it takes longer for Outlook to respond that show this error.