Campaign is not sending email because of sendmail error

Dear Friends,

I have set up outbound emails with SMTP settings only.

But when I am trying to send emails, they are not being sent at all.

suitecrm.log is showing the following error:

01/31/17 01:30:13 [10364][1][FATAL] SugarPHPMailer encountered an error: Could not execute: /usr/sbin/sendmail
01/31/17 01:30:13 [10364][1][FATAL] Email delivery FAILURE:Array
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Could not execute: /usr/sbin/sendmail
(

I have not set up SendMail anywhere, as far as I can remember. However, I have just restored the backup of Website which was running on PHP 5.4. But my current development environment (localhost) is based on PHP 5.7.

Can you provide some solution to this problem?

With thanks,

RK