Hello and welcome.
There are two ways to prevent an app from starting at login:
1. > System Preferences... > Users & Groups > click your account in list on left > Login Items > click Mail to highlight it > click the - button
2. Click and hold the Mail icon in the Dock > select Options from the pop-up > click Open at Login to remove the checkmark.
After either above, restart or logout/login.
Let us know if that doesn't fix it.
C