The ‘api_key: invalid’ error arises when your SMTP plugin and SMTP.com aren’t unable to connect due to an incorrect API key. This API key is essential for your SMTP plugin to utilize SMTP.com’s email services.
Resolving the Error – Solution
To resolve the ‘api_key: invalid’ error, ensure the API key in your SMTP plugin matches the one in your SMTP.com account. To get the right API key, log into your SMTP.com account and navigate to the API settings page. Here, you’ll find the key in the Key column.
If you’re using the Easy WP SMTP plugin, you can do this faster. Go to Easy WP SMTP » Settings and click the Get API Key link. This takes you directly to the API page on SMTP.com.
Next, copy the full value in the Key column, then go back to the Easy WP SMTP settings, and paste it into the API Key field.