SMTP Lost Connection

Hi everyone,
I try to understand why I lost connection for some mail.

SSL_accept error from mail135-13.atl141.mandrillapp.com[198.2.135.13]: Connection timed out
déc. 20 21:55:02  postfix/smtpd[465]: lost connection after STARTTLS from mail135-13.atl141.mandrillapp.com[198.2.135.13]

Any idea ?
Regards