P.S. A way to reproduce (not every time, but often):
delete all files in /var/spool/exim/db , restart Exim with HUP signal,
send a message to @ yahoo.com . After the message is received (with auth)
by Exim, while it is waiting in the queue (for unknown reason):
-rw-r----- 1 mailnull mail 16384 Dec 25 18:25 callout
-rw-r----- 1 mailnull mail 0 Dec 25 18:25 callout.lockfile
-rw-r----- 1 mailnull mail 0 Dec 25 18:25 retry.lockfile
-rw-r----- 1 mailnull mail 16384 Dec 25 18:25 wait-throttled_smtp
-rw-r----- 1 mailnull mail 0 Dec 25 18:25 wait-throttl~mtp.lockfile
root@lena:/root# exim_dumpdb /var/spool/exim wait-throttled_smtp
mta6.am0.yahoodns.net 1eTWxP-000Kok-PE
mta5.am0.yahoodns.net 1eTWxP-000Kok-PE
mta7.am0.yahoodns.net 1eTWxP-000Kok-PE
root@lena:/root# exim_dumpdb /var/spool/exim callout
25-Dec-2017 19:54:35 yahoo.com callout=accept postmaster=unknown random=unknown
25-Dec-2017 19:54:35 lena_kiev @ yahoo.com/<Lena @ lena.kiev.ua> callout=accept
root@lena:/root# host yahoo.com
yahoo.com has address 206.190.39.42
yahoo.com has address 98.139.180.180
yahoo.com has address 98.138.252.38
yahoo.com has IPv6 address 2001:4998:44:204::100d
yahoo.com has IPv6 address 2001:4998:c:e33::53
yahoo.com has IPv6 address 2001:4998:58:2201::73
yahoo.com mail is handled by 1 mta5.am0.yahoodns.net.
yahoo.com mail is handled by 1 mta6.am0.yahoodns.net.
yahoo.com mail is handled by 1 mta7.am0.yahoodns.net.
root@lena:/root# exim -bV
Exim version 4.90 #0 (FreeBSD 6.3) built 24-Dec-2017 20:40:09
Copyright (c) University of Cambridge, 1995 - 2017
(c) The Exim Maintainers and contributors in ACKNOWLEDGMENTS file, 2007 - 2017
Probably Berkeley DB version 1.8x (native mode)
Support for: crypteq iconv() use_setclassresources PAM Expand_dlfunc OpenSSL Content_Scanning DKIM
Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch dbm dbmjz dbmnz dnsdb
Authenticators: plaintext
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile autoreply pipe smtp
Fixed never_users: 0
Configure owner: 0:0
Size of off_t: 8
Configuration file is /usr/local/etc/exim/configure.lena
(I inserted blanks around @ in email addresses in order to be readable
in mailing list archives).
The transport:
throttled_smtp:
driver = smtp
serialize_hosts = *
dkim_domain = <redacted>
dkim_selector = 3
dkim_private_key = /usr/local/etc/exim/dkim3.key
connection_max_messages = 5
max_rcpt = 9
retry_use_local_part
hosts_avoid_pipelining = *
hosts_try_chunking =
