The reason behind the issue is, unfortunately, still not confirm. But, the issue is usually related to one of the accounts/account settings on the iPhone. There can be an outgoing server conflict that might be causing this issue. Majority of the users solved the issue by disabling and re-enabling or deleting and re-adding the problematic accounts. The good news is that the problem can easily be solved.
Method 1: Troubleshoot by Disabling and Re-enabling accounts
This is the method that has worked for the majority of the users. This is basically a troubleshooting method where you disable all the accounts except the one that is having the issue. All you have to do now is to re-enable all the accounts (one-by-one) and send emails to see which account is causing the outgoing conflict. Once you pin point the account, you can then remove and re-add the account. Note: Disabling a mail account isn’t equivalent to deleting the account from iPhone. Disabling an account means that you won’t be able to receive or send any emails from that specific account. Useful for situations where you don’t want to receive emails from a certain account e.g. business account. Here are the steps for disabling the accounts
Once done, do the following
Once the account is deleted, everything should be back to normal. You can re-add the account again and it won’t cause any problems.
Other Tips
These aren’t really solutions but some tips that can help you get to the bottom of the problem. Keep in mind that these tips aren’t real solutions that have known to work for other users. These are simple things that might not have crossed your mind while troubleshooting
If the problem is with an Exchange Account then check if there is another account associated with the Exchange account. If there is another account associated with the Exchange account then check the other account. The sent emails might be ending up in that accountTry to add the account on another iPhone and check if the issue persists. This is to ensure whether the problem is with the account of with the iPhone.Don’t forget to delete the problematic account and re-add it to the iPhone.
Fix: Error “Your Response to the invitation cannot be sent” on iPhone?Fix: oops… a server error occurred and your email was not sent. (#007)Fix: A server error occurred and your email was not sent. (#76997)”Fix: Oops… a server error occurred and your email was not sent (#707)
