This is a bug in FileMaker's use of OLE functions to talk to Outlook.
It is normally resolved by sending quicker and making sure bit numbers of FMP and Outlook match.
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