Kotlin email sending with Gmail: 400 Error - Precondition Check Failed

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1

Gmail API Trouble: 400 Error When Sending Emails in Kotlin

I’m hitting a wall with my Kotlin app. It’s supposed to send emails through Gmail, but I keep getting a 400 error. The message says ‘Precondition check failed.’ Here’s what I’ve done:

  • Set up a service account and stored the key in a JSON file
  • Created a SendMessage class to handle the email sending logic
  • Used the Gmail API client in my code

When I run it, boom! Error 400. It’s driving me nuts. I’ve double-checked my gradle dependencies and they seem ok.

Has anyone run into this before? Any ideas what might be causing it? I’m pretty new to working with the Gmail API, so I might be missing something obvious.

Here’s a simplified version of my code:

fun sendEmail(from: String, to: String): Boolean {
  val creds = GoogleCredentials.fromStream(FileInputStream("key.json"))
    .createScoped(listOf(GmailScopes.GMAIL_SEND))

  val service = Gmail.Builder(NetHttpTransport(), GsonFactory.getDefaultInstance(), HttpCredentialsAdapter(creds))
    .setApplicationName("MyApp")
    .build()

  val email = MimeMessage(Session.getDefaultInstance(Properties(), null)).apply {
    setFrom(InternetAddress(from))
    addRecipient(Message.RecipientType.TO, InternetAddress(to))
    subject = "Test"
    setText("Hello, world!")
  }

  val outputStream = ByteArrayOutputStream()
  email.writeTo(outputStream)
  val raw = Base64.encodeBase64URLSafeString(outputStream.toByteArray())
  val message = Message().apply { raw = raw }

  return try {
    service.users().messages().send("me", message).execute()
    true
  } catch (e: GoogleJsonResponseException) {
    println("Error: ${e.details}")
    false
  }
}

Any help would be awesome. Thanks!

2

I encountered a similar issue when working with the Gmail API in Kotlin. One crucial aspect to consider is the use of a service account. While it’s suitable for many Google APIs, the Gmail API typically requires user authentication. Instead of a service account, try implementing OAuth 2.0 for user authorization. This approach allows your application to send emails on behalf of a specific user, which aligns better with Gmail’s security model. Additionally, ensure you’ve enabled the Gmail API in your Google Cloud Console project and that your OAuth consent screen is properly configured. These steps should help resolve the 400 error you’re experiencing.

3

I’ve been down this road before, and it can be frustrating. One thing that’s not immediately obvious is that the Gmail API has some quirks when it comes to service accounts. Have you considered using a regular OAuth 2.0 flow instead? It’s a bit more work to set up, but it’s generally more reliable for Gmail.

Also, double-check that you’re using the correct scopes. For sending emails, you might need ‘https://www.googleapis.com/auth/gmail.send’ instead of the shorthand version.

Another potential issue could be with the ‘me’ parameter in your send() method. Try replacing it with the actual email address of the account you’re sending from.

Lastly, make sure your email content is properly encoded. Sometimes the Base64 encoding can cause issues if not done correctly. You might want to try using a different method for encoding, just to rule that out as a potential cause.

Keep at it - these API issues can be tricky, but they’re usually solvable with a bit of persistence!

4

hey emma, that error’s a pain! i’ve run into it before. check if ur service account has the right permissions in the gmail api console. also, make sure the ‘from’ address matches the one linked to ur service account. those were my gotchas. good luck!