Are you sure your SendmailModule setting should be Kernel::System::Email::Sendmail and not Kernel::System::Email::SMTPS or Kernel::System::Email::SMTPTLS?
Because if you look into the description of the SendmailModule::Host it states:
If any of the "SMTP" mechanisms was selected as SendmailModule, the mailhost that sends out the mails must be specified.
So, since your system is set to use Kernel::System::Email::Sendmail it's probably just using the system's own sendmail binary and not actually communicating with the external smtp server.
Try changing to either SMTPS or SMTPTLS.
EDIT: Make sure to also change the Port option to 465 or 587 when changing to SMTPS or SMTPTLS