easy-wp-smtp

https://wordpress.org/plugins/easy-wp-smtp/

Configuration with Reseller club Business Email

If using Cloudflare DNS - Make sure proxy for SMTP CNAME is not enabled on Cloudflare.

User: actual email (no alias or forwarding emails)
Encryption: none
Port: 25
Encryption: STARTTLS
Port: 587

Actually the Post SMTP is more feature rich and will do a scan to help you with setup and has built in log system.