SAN FRANCISCO — Hackers targeted SendGrid, a mass email service used by 180,000 companies including Uber, Pinterest, Spotify and Foursquare, to infiltrate Coinbase, one of the most popular Bitcoin exchanges.

SendGrid confirmed that one of its Bitcoin-related clients was compromised on Wednesday. It would not name the customer, but Coinbase confirmed in an email on Thursday that hackers had compromised its SendGrid account, though it said no Bitcoin were stolen.

In a statement, SendGrid said it believed the attack was an isolated one. However, the attack follows a similar pattern to an attack last year against a former SendGrid customer, and there is evidence that other Bitcoin companies are being targeted via their mass email providers.

Mass email services like SendGrid, which sends 14 billion emails a month, are a powerful tool for hackers looking to send spear-phishing emails on a large scale. SendGrid sends transactional emails on behalf of trusted companies like Spotify and Pinterest, alerting customers to updates in the service and new followers. Most customers don’t even realize the emails are coming from SendGrid, making it more likely that they would take the bait and click on malicious code that grants hackers access to their accounts.

Last year, ChunkHost, a fast cloud service provider that accepts Bitcoin and is used by many Bitcoin-related clients, said it was targeted by hackers through its SendGrid account.

Nate Daiger, a co-owner of ChunkHost, said that a hacker had managed to persuade a SendGrid employee to change his company’s account information over the phone, took over ChunkHost’s account and reset passwords for two of its Bitcoin-related clients in an apparent attempt to steal Bitcoin wallets.

Ultimately, Mr. Daiger said, the attack was unsuccessful because both clients used two-factor authentication, a security mechanism that requires a second password when logging into an account from an unrecognized machine.

Mr. Daiger said this was the second time his company was targeted by hackers through SendGrid. When Mr. Daiger tried to warn other SendGrid customers about the potential security hole in a blog post, he said that he was asked by SendGrid to retract his post. He refused. In an email, SendGrid denied ever asking Mr. Daiger to retract the blog post.

In a statement, SendGrid said that it was not the only email service provider that had been targeted in this week’s attempts to steal Bitcoin, and that the compromise at Coinbase, using its service, was an isolated incident.

“From SendGrid’s perspective, this appears to be an isolated attack on one SendGrid customer, however we are aware that users of other Bitcoin related businesses have been targeted this week with phishing attacks via multiple email service providers,” the company said in a statement.

Online, users of two Bitcoin services, including Localbitcoins and Btcjam, complained they were receiving spear-phishing emails through mass email service providers. They did not name the service providers, although one Localbitcoins spear-phishing email appeared to come from Mailchimp, a SendGrid competitor that sends email newsletters on behalf of seven million customers. Mailchimp did not return repeat requests for comment, nor did Localbitcoins or Btcjam.

But the motivation for attacking these companies was crystal clear.

Mr. Daiger said that although hackers were ultimately unsuccessful in stealing Bitcoin, he did not foresee them surrendering their efforts any time soon. “The fact that you could potentially steal thousands or millions of dollars make these attacks pretty motivating!” he said.