When you change timeout property value to -1 it means infinity theoretically for connection and it will not close. Secondly, this associated issue is likely to get resolved in upcoming version of Aspose.Email as well.
Is the time out issue fixed in 19.6 release?
I have observed your comments and like to inform that this issue will be fixed in Aspose.Email 19.7. I request for your patience and we will share good news with you soon.