==== Required information ====
- iRedMail version (check /etc/iredmail-release):  0.9.8 MARIADB edition.
- Linux/BSD distribution name and version: Ubuntu 16.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): Mysql
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? No
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Some additional info
PHP : php7.0-fpm

In php.ini I have:
; For Unix only.  You may supply arguments as well (default: "sendmail -t -i").
; http://php.net/sendmail-path
sendmail_path = "/usr/sbin/sendmail -t -i"

I am able to send an receive email with rounddcube.

When wordpress is trying to send an email via  wp_mail I get an error message:
The email could not be sent.
Possible reason: your host may have disabled the mail() function.