Lob

Physical mail as an API, with a print and delivery network behind it.

An API call that arrives in an envelope.

What it is sold for

Sending personalised physical mail programmatically.

The rest of payments and messaging, alphabetically. Sharing a category is a filing decision, not a relationship.

Ably

Hosted realtime messaging with delivery guarantees and presence, sold on what happens when connections drop.
Anymail finder

Finds work email addresses from a name and a domain, and charges only for the ones it can verify.
Applozic did not answer

Chat and messaging SDKs. The site presents an expired or misconfigured certificate and would not load.
Braintree

PayPal's payment platform for merchants who need cards, wallets and PayPal behind one integration.
EasyPost

One API across many carriers for rates, labels and tracking.
FormSpree

A form backend for static sites, so an HTML form can submit without any server behind it.
Heybounce

Email validation before sending, protecting sender reputation from hard bounces.
Hunter.io

Finds and verifies professional email addresses from a domain, one of the older tools in this category.

ImranTools[Lob]