xasharma the mail
driver relies on the server configuration, the server needs to be capable of sending emails. Considering you mention some emails actually do arrive, then that means that works. The next step is that your SPF and DKIM need to be configured to allow the server, where you site runs, to be approved to send on behalf of the domain. There are a lot of articles on the internet related to this topic and I don't think I could cover every topic sufficiently, so please consult those online articles. Configuring email sending is a hosting task, if you don't know what you're doing get your hosting support desk involved.