You can change the default configuration of OssnMail. OssnMail uses phpmailer so you can add paramaters from phpmailer. One of the example for this is SMTP component.

<?php
function ossn_com_smtp_init() {
		ossn_add_hook('email', 'config', 'ossn_smtp', 1);
}
function ossn_smtp($hook, $type, $mail, $return) {
		//$mail contain the default phpmailer/OssnMail object
		$mail->IsSMTP();
		$mail->SMTPAuth = true;
		//$mail->SMTPDebug = false;

		$mail->Host       = 'host';
		$mail->Port       = 'port';
		$mail->Username   = 'username';
		$mail->Password   = 'password';
		$mail->SMTPSecure = true;
		return $mail;
}
ossn_register_callback('ossn', 'init', 'ossn_com_smtp_init');

One other usecase is sending HTML emails

<?php
function ossn_com_mail_init() {
		ossn_add_hook('email', 'config', 'ossn_html_emails', 1);
}
function ossn_html_emails($hook, $type, $mail, $return) {
		//$mail contain the default phpmailer/OssnMail object
		$mail->isHTML(true)
		return $mail;
}
ossn_register_callback('ossn', 'init', 'ossn_com_mail_init');