Swift-SMTP interoperability problem with Yahoo SMTP for 8k+ attachments

[Darrell]

I'm not sure whether this problem with worthy of your attention, since (so far) it seems to be specific to one email provider and for large attachments only. But I thought I'd report it in case more providers are affected.

When sending an attachment of size 8k+ from my Yahoo account and using smtp.mail.yahoo.com as my SMTP server, I get the following error message:

Bad response received for command. command: (
.), response: 500 Line length exceeded. See RFC 2821 #4.5.3.1.

Sending an attachment of size 4k via Yahoo succeeds.

Sending an attachment of size 8k via Mac/iCloud SMTP infrastructure succeeds.

This is in a MacOS app with the following version information:

Swift 4.2
Xcode 10.1
MacOS 10.14.2
SwiftSMTP 5.1.0
LoggerAPI 1.8.0
Cryptor 1.0.23
SSLService 1.0.44
Socket 1.0.44

Below is my "send email" code showing full parameters from my test MacOS project. I have a test project if that is helpful.

Darrell

@IBAction func sendEmailButton(_ sender: NSButton) {
    scrollTextOutlet.string = ""
    let serverHostname = serverHostnameOutlet.stringValue
    let fromAddress = fromAddressOutlet.stringValue
    let fromPassword = fromPasswordOutlet.stringValue
    let toAddress = toAddressOutlet.stringValue
    let smtp = SMTP(hostname: serverHostname, email: fromAddress, password: fromPassword, port: 587, tlsMode: .requireSTARTTLS, tlsConfiguration: nil, authMethods: [], domainName: "localhost")
    let sender = Mail.User(name: "smtp test email user", email: fromAddress)
    let recipient = Mail.User(name: "test recipient", email: toAddress)
    var mail: Mail
    if attachmentCheckbox?.state.rawValue == 1 {
        self.scrollTextOutlet.string += "using attachment \(self.selectedPath)\n"
        let attachment = Attachment(filePath: self.selectedPath)
        mail = Mail(from: sender, to: [recipient], cc: [], bcc: [], subject: "test email subject", text: "test email body", attachments: [attachment], additionalHeaders: [:])

    } else {
        self.scrollTextOutlet.string += "sending email without attachment\n"
        mail = Mail(from: sender, to: [recipient], cc: [], bcc: [], subject: "test email subject", text: "test email body", attachments: [], additionalHeaders: [:])
    }
    smtp.send(mail) { (error) in
        if let error = error {
            self.scrollTextOutlet.string += "email error \(error)\n"
            if let error = error as? SMTPError {
                self.scrollTextOutlet.string += "\(error.description)\n"
            } else {
                self.scrollTextOutlet.string += "\(error.localizedDescription)\n"
            }
        } else {
            self.scrollTextOutlet.string += "test mail sent successfully\n"
        }
    }
}