Change Mail Server Default for subdomain

Hi, straight forward, I have some questions. I have a cpanel account, example named example.com and i have subdomain named subdom.example.com then I made an email account named username@subdom.example.com I turned on the Auto Discovery. And when I register my email address to Thunderbird Mail Client using IMAP, the default setting for Incoming Mail Server and Outgoing Mail Server are subdom.example.com I still can received email but I cannot send email from thunderbird. But when I change the smtp server in Thunderbird to mail.example.com I can send email I want to set default Incoming & Outgoing Mail Server set to mail.example.com Can I? and How? Is this best practice to implement? Thank You