Email with attachment > 3 MB remains stuck in Outbox when sent via MFMailComposeViewController on iPadOS 26.0.1

I’m seeing a strange behaviour when sending mail from my app with MFMailComposeViewController.

If the attachment is larger than about 3 MB, the message is created but then remains stuck in Outbox on my iPad running iPadOS 26.0.1.

If I open the stuck draft in Outbox and tap Send immediately, the message is sent successfully.

If I wait for a while before resending, the attachment shrinks to a few hundred bytes and the resend goes out without the file (I think the truncation is related to the MIME type, because it is not happening always).

This happens only on the iPad:

• iPhone 16 on iOS 26.0.1 – message sends normally
• iPhone XR on iOS 18.7.1 – message sends normally
• iPad on iPadOS 26.0.1 – message remains stuck in Outbox with attachments over ~3 MB

Mail settings: default iCloud account (also tried explicit setPreferredSendingEmailAddress("…@icloud.com")), Low Power Mode and Low Data Mode are off.

Has anyone else encountered this behavior on iPadOS 26, or found a reliable workaround?

Here’s the minimal repro I’m using:

import UIKit
import MessageUI

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        
        let mailComposer = MFMailComposeViewController()
        mailComposer.setSubject("test")
        mailComposer.setMessageBody("test", isHTML: false)
        
        let repeatedString = String(repeating: "aaaaa aaaaa\n", count: 300000)
        let data = repeatedString.data(using: .utf8)!
                                   
        mailComposer.addAttachmentData(
            data,
            mimeType: "application/octet-stream",
            fileName: "text.txt"
        )
        self.present(mailComposer, animated: true)
    }
}