requirements could not be resolved

[alex9434]

I get the following error message when I try to install with composer require faonni/module-smtp

Your requirements could not be resolved to an installable set of packages.

Problem 1
- Can only install one of: magento/module-email[101.0.0, 100.2.4].
- Can only install one of: magento/module-email[101.0.0, 100.2.4].
- Can only install one of: magento/module-email[100.2.4, 101.0.0].
- faonni/module-smtp 2.3.1 requires magento/module-email 101.0.* -> satisfiable by magento/module-email[101.0.0].
- Installation request for faonni/module-smtp ^2.3 -> satisfiable by faonni/module-smtp[2.3.1].
- Installation request for magento/module-email (locked at 100.2.4) -> satisfiable by magento/module-email[100.2.4].

Installation failed, reverting ./composer.json to its original content.

My sytem is Magento 2.2.6 on ubuntu 18.04 with PHP 7.1