By default, EASendMail SMTP Service always uses default instance to send email. That means all emails sent from EASendMail use the same configuration and queue.
However, if you want to use different settings and queues for different clients, You can use Instance feature.
How to create instance
First of all, please go to EASendMail Service installation path\insts folder, then create a folder with any name, then this folder is a new instance and the instance name is the folder name and you should re-start Emailarchitect EASendMail Service in Administrative Tools -> Services to take effect.
For example, if you create a folder named "myinstance" under insts folder, then you can choose "myinstance" in EASendMail Service Manager and set different configuration for this instance.
Finally, you just need to use "SendMailToQueueEx" method and specify the instance name ("myinstance") to send the email by this instance.
Disclaimer Configuration Headers and Variables SMTP Instances Journal and troubleshooting