On multisite suddenly emails are not working

We have a website on WordPress multisite network. There are few basic forms on the main website as well as on all network sites under the main site. All were working fine. Suddenly the forms on the network sites stopped working for the last few days. Emails are not being sent while submission of the forms but on main site form is working fine. Any reason? The contact form 7 plugin is already updated to the latest version.

Please suggest how to resolve this.