Version: 4.0.2

Bug Description

I have very simple code to send email using SendmailMailer.
Everything is working in nette/mail v3.1.11. When I upgrade to nette/mail v4.0.2 there is error:

Nette\Mail\SendException - Unable to send email

Steps To Reproduce

Try this code with nette/mail v3.1.11 (working) and with v4.0.2 (not working)

$mail = new Nette\Mail\Message;
$mail->setFrom('Franta <franta@example.com>')
	->addTo('petr@example.com')
	->setSubject('Potvrzení objednávky')
	->setBody("Dobrý den,\nvaše objednávka byla přijata.");
$mailer = new Nette\Mail\SendmailMailer;
$mailer->send($mail);

It seems to me, that the problem is on line SendmailMailer.php on line 52 - after removal it work ok.
$cmd .= ' -f' . key($from);

Expected Behavior

Code is working with both versions (v3.1.11 and v4.x.x)