[exim] SMTP command timeout on TLS

I've been searching online and the list for a while but haven't been
able to find an explanation for an error/warning message.

I have exim configured to use STARTTLS and to authenticate a sending
user with SASL. It works, I'm able to send email via my server from a
client with a valid authentication password. However, what I see after
I send a message is the error/warning:

SMTP command timeout on TLS connection from <remote host>

(doesn't matter which remote host, they're all doing it with various
clients). Since the configuration seems to work I haven't really
worried about it but I've been curious about the possible cause and if
it can be fixed. The message shows up long after the email has been
sent (usually five to ten seconds afterwards). Otherwise the exchange
appears to occur normally (I see the saslauthd succeed, the mail is
accpeted then sent on its way to the final destination and exim says
"Completed" and then, after a short delay, "SMTP command timeout..."