There are could be many reasons to have Magento 2 mail function configured via SMTP, but the major are the following:

• You hosting company restrict/disable php mail function (for example CloudWays)
• You want to increase reliability of mail delivery

So if for whatever reason that needs to be done, you will need to find an extension. There a number of SMTP extension available on the market for Magento 2 CE and Magento 2 EE, some of them are paid when the other are free. I did some personal research and found that free Mageplaza Magento 2 SMTP extension does the job very well. It’s easy to install and it support all the core features and plenty of integrations with mail service and providers.

Ready to install? It’s very simple.

Login via SSH to the root directory with your Magento 2 files and execute the following commands one-by-one:

composer require mageplaza/module-smtp
php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy

One it’s done, go to Admin section and finish configuration with your SMTP service/provider.

P.S. I personally tried a number of different SMTP providers and so far I am happy with sendgrid.com They offer free account and works just fine for me.