You are on WIN or MAC - version? FMPA version?
The email support in FMP is very basic.
It can send plain text emails from email client software depending on the email client - Outlook has problems.
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Can FileMaker Pro send email via a SMTP server rather than an email client?
Answer ID: 6991Last Updated: Apr 03, 2012
A Likely Fix for SMTP Send Mail Error 1506 Woes
There are plugins that give much greater control and capabilities. And work with IWP
Threads about sending emails on these forums.
There are email management services online that can be connected to filemaker (have not done this)