The IMAP/SMTP settings are configured when creating your email credentials within the 'Manage Apps' section. The app itself uses PHPMailer (https://github.com/PHPMailer/PHPMailer) in order to send out the emails and uses the credentials you supply directly. Depending on your OS/platform, you shouldn't need to install any php mail module as PHPMailer will either use the sendmail binary (if it's found on your system otherwise it will use Postfix, QMail etc.) or it's own mail functions. Postfix is only necessary is sendmail isn't available and you are sending via the 'System Default' emailing method.
Regarding the ability to verify credentials, did you save the credentials then try to send email via those credentials? I have run into an instance where the button was not working properly but that bug should have been fixed in version 6.9.
To receive incoming email AKA to use the app as a mail client, you must have credentials set for an IMAP server as the app uses IMAP to retrieve email. If you are looking to record incoming email messages with their respective contacts, please refer to the 'Email Dropbox' configuration documentation found here: http://wiki.x2crm.co...eceiving_Emails
Please feel free to bump this thread with further questions regarding email.