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Lost in all the technical jargon? Find definitions for common email and technical terms and learn more with our related resources.


REpresentational State Transfer (REST) is an architectural-style Application Programming Interface (API) that centers around manipulating resources. RESTful APIs boil down to a simple request-and-response pattern. Brands interact with a RESTful API by using a program that uses the API to request data from the server. Then, the API returns data from the server, and the client displays the results to the user. These requests are generally made using the Hyper-Text Transfer Protocol (HTTP).


ROI, or Return on Investment, is a metric used to determine an investment’s efficacy, calculating whether it has earned a profit or incurred a loss. When expressed in a formula, it accounts for the profit or loss divided by the total investment and multiplied by 100. It’s expressed as a percentage. Here’s how:

Rate limiting

Rate limiting is a technique to restrict traffic on a server. In email terms, rate limiting in Application Program Interfaces (APIs) refers to how an email API might limit the number of calls an organization can make during a set period, impacting the number of emails they can send during that time frame. Rate limiting is a way to ensure fair use and is also an excellent way to protect infrastructure from hackers who might spam an open API endpoint on a brand’s website with too many requests. Organizations typically rate limit web app APIs to secure their websites from cyber attacks and manage overall network traffic.

Reactivation email

A reactivation email, or re-engagement email, is sent to users no longer engaging with a brand (usually after a period of about three months). Even if users initially sign up to receive the brand’s communications, they may change their minds over time and stop opening marketing emails. This is when the brand can entice them back with promotional coupons, discounts, and offers.


A redirect is a method to divert recipients to a web page different from the one they initially intended to visit. This could happen due to a broken original web page or a change in the web page’s URL.

Return path

A return path is used to specify where bounced emails are sent and is placed in the email header. It’s an SMTP address separate from the sending address. This is a good practice for email delivery, as it gives bounced emails a place to land – other than in an inbox – making it easier to avoid sending notifications to bounced addresses. However, it’s important for authentication to ensure that the return path domain is the same as the sending domain.

Ruby programming language

The Ruby programming language is an open-source, object-oriented scripting language that leverages reusable code to speed up development. Its applications include: