Mailgun, an email startup, spins out of San Antonio-based Rackspace and announces it has raised $50 million.

Mailgun, based in San Antonio has offices in Austin and San Francisco.

Turn/River Capital led the investment round with participation from Scaleworks and Rackspace. The company plans to use the money to accelerate product development and expand customer support.

Mailgun, which is built for developers, lets them send, receive and track email easily from within their applications without managing an email server. Customers include development teams at Slack, GitHub, Stripe, Lyft, OfferUp, Zapier and Cinemark.

William Conway will lead the new startup, Mailgun, as CEO. He previously served as general manager of Mailgun inside Rackspace.

“Developers are at the heart of everything we do. We are focused on solving the daily problems developers face when integrating and managing email inside of their applications. Our independence and influx of growth capital will allow Mailgun to drive a product vision that will thrill developers when they see what is coming down the pipe,” Conway said in a news release. “Turn/River Capital’s experience growing similar developer tools makes them a perfect match to help drive Mailgun’s continued growth.”

Rackspace acquired Mailgun in 2012. The company was part of San Francisco-based Y Combinator’s 2011 accelerator program.