After collaborating with a partner on a support request we have identified an interesting behavior related to the customers using e-mail account of type "Shared Mailbox" for sending emails from Business Central, however the sent email is stored to the "Sent emails" folder of the current user, instead of the folder of shared mailbox. 

Furthermore, when users send email manually from outlook client the sent email is store correctly in shared mailbox folder.

I believe it would be a great idea if. The sent email is stored in user mailbox instead of shared mailbox. To see all sent emails from the shared mailbox in the shared mailbox

Current workaround found is the following: create a transport rule where the sender is SMTP user account and then a redirect rule via Add Recipient which is the SMTP user account. From Outlook or OWA create a rule that moves messages to sent Items folder if they come from SMTP user account.