If your Outlook suddenly stops and start defining an error message on the screen in a instead of sending emails properly. As the reports say that Outlook Error 0x800CCC78 is counted as most typical error while working with Microsoft Outlook. This error message notifies about the wrong inputs of the outgoing sever detail. Remember, this error does not impact upon incoming of Outlook messages. If you will search over the internet about this error, then you will find that most of Outlook users have stated that in such circumstances they are able to receive emails rather than sending messages.
Outlook Error 0x800CCC78: Unable to send message. Please verify the e-mail address in account properties. The server responded: 530 5.7.0 must issue a STARTTLS command first.'(0x800CCC78): 'Outgoing SMTP does not support SSL...' - currently my SMTP port is set to 587, IMAP port 993 both have 'This server requires an SSL connection' checked yes.
Cause1: ISP or Internet Service Proivder defines port values for the synchronizing of networks in a domain and through which sending or receiving of data could be possible. Likewise, to transfer emails from Outlook we used some port numbers and 25 port is default which has found as most usable port number. But, this goes wrong when ISP blocked this port number and changes it to another one for a domain. Outlook Error 0x800CCC78 is caused when 25 port number gets blocked by ISP and 587 is enabled in place of it.
Cause 2: Outlook Error 0x800CCC78 also raises up due to missed authentication on outgoing server settings. Non-authenticated outgoing details do not allow emails to move further on the network and stop it from where it was actually started (email client-Outbox). So, always check "My Outgoing Server (SMTP) Requires Authentication", if you have not enabled it.
If in case the given solution does not help you in resolving Outlook error 0x800CCC78, then you should switch to an effective professional tool like Outlook Data Recovery that can help you in getting rid of all annoying errors and retrieving PST files.