You will need to give us some context about what feature you use, how you set things and which plugin you use to send an email, is it inside a form? or something else?
As I do not have any URL from your website and context I will resort to general troubleshooting steps:
- Ensure everything is up to date according to our version compatibility list here. Please follow the best practices when updating your theme and plugins. Click here for more information.
- If you're using a CDN, please clear the CDN's cache and disable optimization services.
- Test for a plugin conflict. You can do this by deactivating all third-party plugins, and see if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.