Delivery servers are the backbone of MailWizz EMA, without them the application becomes a read-only application without any real purpose.
When we talk about delivery servers, we mean SMTP servers or any other service that has support for a SMTP interface(that includes Amazon SES, Sendgrid, ElasticEmail, Mandrill, etc)

Almost any email account can be used as a delivery server Yes, this is correct, you can use your gmail, yahoo, etc account as a delivery server for your emails and these will work as long as you provide proper login credentials.
Please keep in mind that when using gmail/yahoo/etc, basically any free email account, there are limits that apply on the number of emails you are allowed to send daily, make sure you configure your server with the proper limits.

MailWizz EMA is designed to work with an unlimited number of delivery servers which is good for you because this means you can have multiple free accounts with multiple email providers and use them all in a centralized place.
When you add a delivery server, you will be prompted to confirm it. The confirmation is needed mainly so that MailWizz EMA can decide if the delivery server is reliable for sending or not.

After you successfully add a delivery server, that server can be used to deliver all you emails and campaigns.
If you have multiple delivery servers, they will be used depending on their probability and their hourly usage quota.
If a server reaches the hourly quota(that you can set when adding it), then that server will not be used for sending for that particular hour, then, in the next hour, the limit is raised.
All these mechansims are in place to avoid you getting banned from various providers.
Of course, if you have your own SMTP server then you have no limit to worry about.

VPS/DS owners If you are a VPS or Dedicated Server owner and you don't have your own SMTP server, then you might give iRedMail a try.
iRedMail can be installed into your server in a few minutes, very easy, and you will end up with a fully functional email server.

Bounce servers association You can associate a delivery server with a bounce server and a bounce server can be associated with multiple delivery servers.
Please see the Bounce servers section to understand what these servers are.