SwiftSMTP broken: Error ioOnClosedChannel on latest macOS

Hi!

I wrote an internal used backup command line tool which is in use since several years.

Today I got an error while sending an email: “Failed: ioOnClosedChannel”.

I assume that the latest macOS updates did break my app. On the server I use macOS 15.7 and on my development machine macOS 26.

Here is the related code:

private func sendMail() {
    var a : [Email.Attachment] = []
    if self.imageData != nil {
        switch self.imageType {
        case .tiff:
            a.append(Email.Attachment(name: "Statistics.tif", contentType: #"image/tiff"#,      contents: ByteBuffer(bytes: self.imageData!)))
        case .pdf:
            a.append(Email.Attachment(name: "Statistics.pdf", contentType: #"application/pdf"#, contents: ByteBuffer(bytes: self.imageData!)))
        case .unknown:
            fatalError("Unimplemented attachment type!")
        }
    }
    mailHtml = mailHtml.replacingOccurrences(of: "<br>", with: "<br>\n")
    
    let email = Email(sender: .init(name: "Backup", emailAddress: "SENDER@MYDOMAIN"),
                      replyTo: nil,
                      recipients: recipients,
                      cc: [],
                      bcc: [],
                      subject: self.subject,
                      body: .universal(plain: self.mailText, html: mailHtml),
                      attachments: a)
    let evg = MultiThreadedEventLoopGroup(numberOfThreads: System.coreCount)
    let mailer = Mailer(group: evg,
                        configuration: smtpConfig,
                        transmissionLogger: nil) 
    do {
        print("Sending mail... ", terminator: "")
        try mailer.send(email: email).wait() // <-- ERROR HERE Failed: ioOnClosedChannel
        print("done.")
    } catch {
        print("Failed: \(error)")
    }
    do {
        try evg.syncShutdownGracefully()
    } catch {
        print("Failed shutdown: \(error)")
    }
}

I use https://github.com/sersoft-gmbh/swift-smtp.

Any clue about the reason of this error?

TIA,

GreatOm