To check/change the default calendar
1.) On the main screen choose Settings
2.) Then choose Mail, Contacts, Calendars
3.) Scroll down until you see the Mail section and look for a option called Default Calendar.
4.) Once you selected Default Calendar make sure the calendar under the appropriate email account is checked.
To check/change the default mail account
1.) On the main screen choose Settings
2.) Then choose Mail, Contacts, Calendars
3.) Scroll down until you see the Mail section and look for a option called Default Account.
4.) Once you selected Default Account make sure the account desired account is checked.