SMTP Host of Plesk Onyx?

Can anyone tell me what is SMTP Host of Plesk Onyx?
mail.domain.com or webmail.domain.com ?

Thanks in advance.

Hello @David Beroff,
In fact both of them will work, because any domain or subdomain pointed to your server IP can be used a SMTP address.

But it's highly recommended to use SSL/TLS connection between your mail client and your server, and you will probably get issues or warning by using mail.domain.com or webmail.domain.com.
Because Plesk only support configuration of a single SSL certificate to secure the mail server. So, if you choose mail.domain.com, and use Let's Encrypt to issue a SSL certificate, it will work for this domain, but for all other domains (eg mail.example.net), you will have warning about the SSL certificate.

The best solution is to use the same IMAP/POP/SMTP address for all sites hosted on your server and to issue an SSL certificate with Let's Encrypt for this address.
I recommend to my customers to use the server hostname to access to the Plesk web interface and as mail server address. For example, if your server hostname is plesk.yourdomain.tld, you can use it as IMAP/POP/SMTP host for all sites on your server.

I am using WP Mail SMTP wordpress plugin, it ran on cPanel hosting but it didn't run on Plesk hosting when using same configurations.

What to change (in the image below) to allow emails send/receive on Wordpress?

Trying with TLS, I think Plesk will work with this option.

if that is for a domain hosted on that server... you can use "localhost" or the server hostname or the domain name of the wordpress site. those all should work.

Hello, greetings

Make sure there are correct DNS records for following,
mail.domain.tld A <IPv4>
domain.tld MX mail.domain.tld
domain.tld TXT v=spf1 a mx ip4:xxx.xxx.xxx.xxx -all
DKIM
DMARC

In case your server is setup to only communicate over Encrypted connection;
SMTP Host: mail.domain.tld
Encryption : SSL
SMTP Port: 465
Authentication : On

Refer for additional understanding : https://support.plesk.com/hc/en-us/articles/115000424274

Also, make sure SMTP port is open in firewall and not blocked by ISP [In some cases port 25 is blocked by ISP].
In case of custom SMTP port; do check your mail server configuration.

Regards