Outlook 2013 not sending emails: Annual problem for August?

Outlook 2013 not sending emails: Annual problem for August?

Hi there

I run Outlook 2013 as a mail client. It's been working fine, sending and receiving emails.
But as of Yesterday, the 4th of August 2015, it has stopped sending them. It can receive them, but fails to send.

Please could you investigate this issue?

Regards

Ed NR