[exim] callout/defer_ok and smtp timeouts

Should a recipient verify with defer_ok and callout=10s, which gets a
TCP connection but times out during the SMTP conversation, return
success?

         deny    message =       recipient does not appear to exist
                 !verify =       recipient/callout=20s,use_sender\
                                         ${if eq {$sender_address}{}\
                                                 {}{/defer_ok} }

I'm seeing this deny firing (on a non-null sender). Exim 4.52
(but the docs appear similar for 4.62).

- Jeremy