The email addresses that MemberMouse makes available are those of the Employee Accounts you have setup in General Settings > Employees. In some cases you can choose which email address is used, in others the default account is used. The default employee account is marked with the icon.
The Welcome Email's 'From:' address can be changed by editing the membership level in Product Settings > Membership Levels > Edit (pencil icon in the Actions column) > Welcome Email > and selecting the desired address in the 'From' dropdown box.
The Push Notification's 'From:' address can be changed from Developer Tools > Push Notifications > Edit (pencil icon to the right of the Status column) > and selecting the address in the 'From' dropdown box.
Reset Password emails will be sent from the default employee email (as indicated by the orange flag) in General Settings > Employees.