To set up an SMTP account for each individual account in CPanel.
I see the need to configure SMTP sending through personalized accounts for some customers.
There should be an option within each account CPanel where you can set your own SMTP server and all emails from that account are sent through that SMTP.
So when there are customers who require emails sent from webmail use their SMTP data, they can send them without any problems.
Thus each client could put custom data services, such as: SendGrid, mandrill, etc ..