Webmail Issue Sending and Receiving!

I have this slight problem when sending an email from webmail and receiving emails from other servers or senders!

Roundcube Webmail Compose

I don’t use SMTP for the start and when tried over and over it shows same error!

Any ideas how to solve this? If needed to check error log(s), where is that file or folder to look at??

Check /var/log/exim4/mainlog