Thank you for writing to Aspose support team,
Aspose.Email for Java works fine in many environments, however we have not used it in Spring Boots Executable Jar.
You may please try some other features of the library too, like working with MailMessage or PST handling etc. and check if it hangs there as well. If not then share the code which you have tried in this environment using Aspose.Email for Java.
If issue is not resolved, then please assist us to re-produce this scenario by providing steps in detail. These steps should provide details of configuring the environment and then debugging to the line where issue occurs. If possible send us the sample project as well that can be used to observe the issue.