Unable to send an email from a shared mailbox with a different domain

Hi,

I have a single O365 tenant with multiple domains.

I have active users with the first domain eg john@domain1.com 

I created a shared mailbox and had to use the workaround to create by adding a john2@domain2.com,  I then added an alias john@domain2.com  and made this primary, and then deleted the original john2@domain2.com .

I have set Read and Manage and Send As permissions to john@domain1.com , and have set sent items copied to mailbox 

I can email john@domain2.com,  and can successfully reply, the reply is delivered and the mail is displayed in sent items

However, if I write a new email and change the From by selecting john@domain2.com  from the Global Address List the email appears as sent with no error, but is never delivered.

I have read through a number of similar queries and fixes but still cannot get this to work.

Many thanks