Office 2016 Local Exchange and 365 Account

[ITGURU]

I now have 2 email addresses with working across 2 schools. I want to combine both into Outlook.

One is a local exchange account (primary base) and the other office 365.

I can add the 365 account into Outlook as an additional account so both appear down the left hand side.

 

However, if I send an email from the local exchange account all the emails get stuck in the outbox and don't send.

 

Any ideas why this would be?

 

 

Currently, i'm using Outlook for main email and Windows 10 Mail App for the 365 account.