https://youtu.be/B-CL4IEty74

SendGrid is a cloud-based SMTP provider that allows you to send email without having to maintain email servers. SendGrid manages all of the technical details, from scaling the infrastructure to ISP outreach and reputation monitoring to whitelist services and real time analytics.

Send Email From Your Application

SendGrid provides two ways to send email: through our SMTP relay or through our Web API. SendGrid provides client libraries in many languages. This is the preferred way to integrate with SendGrid. If you choose to use SendGrid without a client library, the Web API is recommended in most cases as it is faster, provides some benefit with encoding, and tends to be easier to use. SMTP provides many features by default, but is harder to setup.

Web API
• The Web API has some advantages over SMTP:
• If your ISP blocks all outbound mail ports and your only option is HTTP.
• If there is high latency between your site and ours, the Web API might be quicker since it does not require
as many messages between the client and server.
• If you do not control the application environment and cannot install and confgure an SMTP library.
• If you build a library to send email, developing against a web API provides quicker development.

SMTP Relay
• If you are integrating SendGrid with an existing application, setting up the application to use our SMTP relay
is easiest, as it only requires modifying SMTP confguration.
• Change your SMTP username and password to your SendGrid credentials
• Set the server host name to smtp.sendgrid.net
• Use ports 25 or 587 for plain/TLS connections and port 465 for SSL connections

Helpful Information

For most users we suggest port 587 to avoid rate limits set by some hosting companies.

With SMTP, 100 messages can be sent with each connection. 

SendGrid extends SMTP with the X-SMTPAPI header, giving you more control over how SendGrid sends your email. See the SMTP API documentation for more information.

Customers should utilize SMTPAPI if this is an option. As with SMTP, 100 messages can be sent with each connection, but there can be 1000 recipients for each message.

We strongly discourage users from sending mail directly through a single specific IP address when integrating with SendGrid.

Always point your traffic to smtp.sendgrid.net.

Source: Sendgrid