Windows 10 Pro (15.5.8801.3) same issue also professional perpetual license.
Using a simple custom smtp server (port 25) to our exchange 2010, upon testing the "new email settings interface" I receive the 'No compatible authentication mechanisms found'.
3cxSystemService.log
2018/02/14 16:29:46.208|7040|0007|Excpt|System.NotSupportedException: No compatible authentication mechanisms found.
at MailKit.Net.Smtp.SmtpClient.Authenticate(Encoding encoding, ICredentials credentials, CancellationToken cancellationToken)
at MailKit.MailService.Authenticate(String userName, String password, CancellationToken cancellationToken)
at GenericUtilsNet.Mail.SmtpConnect(SmtpClient smtpClient)
at GenericUtilsNet.Mail.Send(AlternateView calendarData)
at _3cxSystemService.VoiceMailScanner.EmailNotif.SendMail()
at _3cxSystemService.VoiceMailScanner.BoxesMonitor.ProcessList(List`1 list, Dictionary`2 problematicDirectories, ManualResetEvent stopEvent)