SMTP Server (mail server) is a service that allows you to send emails in bulk, fast speed that is not limited to free mailboxes of Gmail or mail hosting.
The SMTP Server application has many, you can use to send Email Marketing, send notifications in WordPress … in general all email related operations need SMTP Server.
Personally, I prefer to use external SMTP Server services. First, it reduces the load for Web Hosting/Server because it does not have the task of sending mail. Secondly, because IP Shared Hosting is often blacklisted by spam mails, so the mail is sent to the Spam. (You do not spam mail but other people use hosting to send spam so you are affected).
Some providers have only SMTP Server service (or transactional email), but there are also providers that support a comprehensive email solution by adding the Email Marketing service.
Top 10 SMTP Server services
SparkPost allows you to send extremely large emails, up to 15,000 free emails per month. If you use more than 15,000 emails, you will have to upgrade to a paid plan, starting at $9 per month with 50,000 emails.
Dedicated IP here is $20 per month, realtime statistics supporting.
Mailgun allows you to send 10,000 free emails per month, from email 10,001 onwards for $0.0005/email. This is a service provided by Rackspace, which is primarily used by developers with rich API libraries.
Mailgun has full tracking statistics (open, click, delivery time, etc), support inbound routing, dedicated IP has official plugin for WordPress.
New accounts are limited to 100 mails/hour. If the delivery rate is good, the limit will be removed. In addition, Account Free can only send to Authorized Recipients, so this account is almost useless. You need to upgrade account by adding card information, will not pay if the number of emails sent every month <= 10,000 mails, beyond this amount to be charged.
SendGrid is also a very famous name in the world. It allows you to send 40,000 free emails in the first month, then if you still use it for free it will limit 100 emails per day. If you exceed this limit, you will have to pay $9.95/month to be sent to 40,000 emails.
Similar to Mailgun, SendGrid also supports full statistics (open, click, delivery time, ect), with plugins for WordPress and Dedicated IP support is cheaper than Mailgun.
SendinBlue allows you to send 9,000 free emails per month, unlimited number of contacts, has tracking and special offers email template library to send professional Email Marketing.
At $7.37/month you can send up to 40,000 emails, dedicated IP for $145/year.
Not only does SMTP Server provide services, Elastic Email also aims to provide all-in-one Email Marketing solution with 150,000 free emails per month. The number of free emails sent daily is 5,000 emails. The next email is $ 0.09/1,000 mail sent.
Full support for Campaigns, Automation, Report (Submitted, Delivered, Opened, Clicked, Unsubscribed, Bounced …), Surveys.
Mailjet offers an all-in-one email service, from Email Marketing to Email Transaction. Mailjet allows you to send 6,000 free mails/month, limited to 200 emails/day. If you want to use more you need to upgrade to higher plan.
Pepipost allows to send 25,000 free emails per month. The next email is priced in a very strange way, based on the number of unread emails for $1 per 5,000 emails.
Postmark allows you to try out 25,000 free emails, then calculate the price based on the number of emails sent.
SendPulse allows you to send 12,000 free emails per month with the SMTP Server service, and with Email Marketing you get up to 15,000 free emails per month (2,500 subscriber limit). If you pass this number you will have to upgrade to higher plan, $8.85 per month for 25,000 emails sent with SMTP Server.
Maildocker is a new provider, supporting you to send 10,000 free emails per month. If you want to upgrade, Maildocker has a lot of flexible plans to suit your needs.