Submission proxy warning connecting to Exim

[Alec Moskvin]

Hello,

When connecting to Exim 4.94, dovecot submission logs this warning:

  dovecot[25235]: submission(alec)<25347><QNydV8+t+oMKAgAF>: Warning: smtp-client: conn localhost:25 ([::1]:25) [1]: Received invalid EHLO response line: Unexpected character in EHLO keyword

Looks like it doesn't like the underscore in "X_PIPE_CONNECT", since
it's neither "isalnum" nor a dash [1]:

  1598480923.401085 220 myhostname ESMTP Exim 4.94 Wed, 26 Aug 2020 18:28:43 -0400
  1598480923.401906 250-myhostname Hello localhost [::1]
  1598480923.401906 250-SIZE 52428800
  1598480923.401906 250-8BITMIME
  1598480923.401906 250-PIPELINING
  1598480923.401906 250-X_PIPE_CONNECT
  1598480923.401906 250-CHUNKING
  1598480923.401906 250-STARTTLS
  1598480923.401906 250 HELP
  1598480923.409242 250 OK
  1598480923.434743 250 Accepted
  1598480923.578335 250- 511 byte chunk, total 511
  1598480923.578335 250 OK id=1kB3uN-0006ar-HU

Adding the line "pipelining_connect_advertise_hosts =" to Exim's config
fixes the issue by disabling X_PIPE_CONNECT.

Is this a bug in Exim or is Dovecot being too strict?

Thanks,
Alec

[1] https://github.com/dovecot/core/blob/2.3.11.3/src/lib-smtp/smtp-syntax.c#L250