Hi, could you please mention how you configured that (assuming an outgoing smtprelay configuration here)?
One solution then would be listing the external email provider’s smtp server(s) in mynetworks, as already posted by @Aleks.
Another solution yunohost hints about, that would even work without using an external smtp server, would be to let yunohost send from a static IP by using a VPN, but I don’t know how to configure that.