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Message Bus has closed a fresh $4 million round of funding to expand its custom email platform, the startup announced today.

Message Bus offers a custom email platform that helps its clients get more engagement from email marketing messages and other customer interactions. Unlike other email marketing services, Message Bus doesn’t rely on ISP whitelists or IP certification but uses real-time analytics and behavioral data to ensure messages get through to (and opened by) recipients. Its clients include About.me, DocuSign, Iterable, LinkedIn, Medium, and MyFitnessPal.

The startup told VentureBeat it plans to use the additional capital to expand development of its email platform, grow its engineering team, and hire more people to handle marketing and sales.

“We see that the most growth is in the midmarket — companies for whom email is core to success, who must dedicate time and resources to it, but who don’t know where to go to get world-class service at a price they can afford. They all start off with existing products such as Sendgrid, Mandrill, Mailgun, and so on, but for similar reasons inevitably outgrow them,” Message Bus CEO Paul Midgen told VentureBeat in an emailed statement.

Midgen added that once mid-level companies outgrow their email platforms, they’re left with only costly enterprise-focused SaaS alternatives. Message Bus, he said, plans to fill the gap by offering an affordable, flexible alternative for managing email messages.

The new round was led by True Ventures, with participation from previous investors North Bridge Venture Partners and Ignition Partners.

Founded in 2010, the San Francisco, Calif.-based startup has raised a total of $18 million in funding to date.