While sending email from WordPress's contact form, I was getting below error -
Failed to send your message. Please try later or contact the administrator by another method.
1. Please make sure you're using your server's name or IP address as server hostname in your contact from configuration. Because contact form used to work from server where your site's A record is pointing.
2. Please make sure you're using only one plugin for configuring the Contact form for your website. Because if you'll configure multiple plugins then there will be conflict between them and will cause the provided error. So disable all other unnecessary plugins related to contact from.
3. Make sure you've entered the correct login credentials that is email address and it's password in your contact form configuration.
It worked for me!