This problem occurs when trying to send a message via an IMAP account configured with SSL/TLS. Problem does not occur when using port 25 with accounts having no encryption enabled. Below is the error message displayed by GO:
Expected response code 250 but got code "", with message ""
++ Starting Swift_SmtpTransport
<< 220 MYSERVER.COM ESMTP IdeaSmtpServer v0.70 ready.
>> EHLO int2.MYSERVER.COM
<< 250-MYSERVER.COM Hello 31-178-10-252.isp.com [188.8.131.52], pleased to meet you
250-AUTH PLAIN LOGIN
>> AUTH LOGIN
<< 334 VXNlcm5hbWU6
<< 334 UGFzc3dvcmQ6
<< 235 2.7.0 User pr.myaccount logged in.
++ Swift_SmtpTransport started
>> MAIL FROM:
<< 250 2.1.0 Sender address accepted
>> RCPT TO:
<< 250 2.1.5 Recipient address accepted
<< 354 Go ahead...
!! Expected response code 250 but got code "", with message ""