Can not configure e-mail (SMTP)

There are 2 areas to investigate:

1. That your site email settings are correct. If you’re using a professional email like Google Workplace then follow the SMTP plugin’s instructions for generating the necessary API. If you’re using self hosted email (and you really shouldn’t be given your knowledge level) doublecheck your mail server settings. If the send settings are bad, your email might get bounced.
2. It might be a problem with your email itself. Doublecheck your mail server settings, domain keys and SPF. It looks like you have zoho mail attached to your domain. There seem to be some missing DMARC security settings. You should contact them to resolve the issue.

“conf error” sounds very general and could also be related to something completely different. Can you give a concrete example? Where exactly does the error message appear and in what context?

Yes there is Zoho mail attached to domain. Today I tried their own plugin (Zoho plugin) to configure email but still no luck. I copyd from API client id, client secret and other stuff but it says “invalid client secret”. I double checked everything. I also read the manual from their page. Still nothing. So wrote letters to Zoho client help. I hope they answer me. If not I am not sure what to do.

Whatever plugin I use, I have checked SMTP plugin’s instructions for generating the necessary API but it always says just “configuration error” or something like that but nothing specific.

Not every SMTP connection requires API credentials. The easiest way is to try it with your own email account – whatever it is, you will certainly have SMTP credentials. Use WP Mail SMTP and enter the data you know for your own email account. If this doesn’t work, your hoster has probably blocked SMTP connections. Then you should contact their support.