An SMTP relay is used to send email between SMTP servers, when the servers are located on different domains.
An email SMTP relay service is used every time you email someone outside your address' domain. SMTP, or Simple Mail Transfer Protocol, does the heavy lifting of sending the message. The relay service moves the message from your email server to the email server of the recipient. It's like the handoff of the baton in a relay race.
A relay is not used when an email is sent between parties on the same domain, because there is only one server involved. However, if there are multiple domains involved between email messages, then a relay is required to deliver between each server.
When you're sending a large volume of email, that handoff—the SMTP relay—becomes very important. If you're sending marketing email like newsletters or transactional email from your applications, you want to make sure that other servers trust that your server is sending good email. An email SMTP relay service is a trusted third party that adheres to sending best practices to help emails get delivered.
In an effort to reduce spam, many internet providers will limit the amount of SMTP relays it will conduct each day. This means that a business that sends large amounts of email may get their legitimate emails categorized as spam. Even a peer-to-peer email server will experience this when sending bulk email messages. An SMTP relay service provider can help prevent this deliverability problem and ensure that your emails get to the inbox.
SendGrid provides SMTP relay services for both transactional email and marketing email. Because we send billions of emails per month, our service is highly scalable, trusted, and reliable. Avoid the negative pitfalls by using SendGrid for automated and bulk email sends.