If you are sending invoices to your customers from your iOS device and each email is sending multiple times, this is likely happening because your Microsoft email account (e.g. hotmail.com, msn.com, live.com, etc.) was setup on your iOS device in the default mail client under the section "Other." All Microsoft emails should be setup under the "Outlook.com" option rather than through "Other."
Please follow the troubleshooting steps in this article to resolve your emailing issue.
iOS and iPhone
1. Go to your iOS device settings
2. Tap Mail, Contacts and Calendars
3. Under the Accounts section, select email account that is sending duplicates
4. Tap Delete Account
5. Confirm that you want to delete email from your device
6. Under the Accounts sections, tap Add Account
7. Tap Outlook.com
8. Enter email and password credentials for email account
9. Tap Yes to let the default iOS mail app have access to your email account
10. Tap Save
11. Under the Accounts section, tap Fetch New Data
12. Enable Push
13. Make sure Fetch schedule is set to anything but Manually