Filled In 1.7.3 is an excellent contact and other form manager for WordPress, leagues ahead of troublemakers like C-Forms II. Still, there are some issues with Filled In on account of its use of Swift Mailer for outgoing email functionality.
Swift Mail has too much functionality and hence is too buggy. Rather than waste a bunch of time fighting with Swift Mail errors, we've replaced Swift Mail with PHP Mailer which is just 40 KB of code instead of 150 KB of code. For the moment, the SMTP and file attachments are not fully tested yet so any responses are warmly welcomed.
Thanks to John for writing modular code which can be repaired.
Specific Bugs of Swift Mail Library
Swift library does not work on some servers and also sends some improper headers which can cause trouble, as for instance with the Relenta CRM e-mail API). Using PHPMailer is more preferred way, since it just works.
Download Filled In 1.7.3 PHP Mailer version here.