Compatible Email clients are listed in FileMaker Help. Look up Send Mail and you'll get the list for your version of FileMaker and for each platform. (The list is different depending on what OS you are using.)
The Help file for FM 11 advanced says in the 'Notes':
To send email, you must have an internet connection. In addition, to send mail through an email application, you must have the following configurations.
A [Mail] section in the Win.ini file.
Microsoft Exchange, Microsoft Outlook, Microsoft Outlook Express and Eudora installed and configured properly.
Since my client with the issue uses Windows, one of Microsoft Exchange, Microsoft Outlook, Microsoft Outlook Express, or (not 'and' as shown in the Help file) Eudora must be configured. I assume then that no other email client is supported. Am I reading that correctly?
That's how I read it. Other email clients may work, but FileMaker didn't test them and thus doesn't list them.
Your other option may be to use SMTP email option though it has its own unique set of problems.
For More Information on an issue with SMTP email, see: Email header malformed when using SendMail
This is one of many acknowledged bugs that can be found in the Known Bug List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html