You can use the Mandrill's Outgoing Server (SMTP) Setting in the MailClient of your choice.
SMTP Settings
Changing the MailClient is not probably going to mark your mails as legitimate.
There are multiple reasons for mail going to spam folder of recipient like:
- Mandrill's IP address got blacklisted in recipient's SMTP (MX) server.
- Your mail contains some links or content which is recognized as suspicious and hence recipient marks such mails as spam.
In short, mail getting marked as spam depends upon:
- Sender SMTP server's reputation.
- Sender's mail content.
- Receiving SMTP (MX) (i.e recipient's) server business logic for mail classification.