I would like to ask the following, because it is a stand-alone version of PC software and there is no intermediate server, I want to build the function of sending email with smtp in the software to facilitate users to send feedback. There are the following questions.

If the email address and password are written in the
1. code, will there be a risk of password disclosure? I compiled
with Qt CPP
2. which free mailbox service is better, which can be sent by domestic and foreign users?
3. will it be stopped by spam if I send too many messages?
4. is there such a third-party service? I see a lot of services for mobile applications.