Appears some change between 2.2.6 and 2.2.7 altered the response codes for LMTP user verification probes.
Dovecot 2.2.6:
Nov 2 15:50:48 spectre postfix/qmgr[627]: 3dBjr80wMgz1s: from=double-bounce@spectre.leuxner.net, size=271, nrcpt=1 (queue active) Nov 2 15:50:48 spectre postfix/cleanup[6226]: 3dBjr80xbYz1w: message-id=20131102145047.2D3C6824147@sam.dfn-cert.de Nov 2 15:50:48 spectre postfix/lmtp[6228]: 3dBjr80wMgz1s: to=tlx@leuxner.net, orig_to=security@leuxner.net, relay=spectre.leuxner.net[private/dovecot-lmtp], delay=0.03, delays=0/0.01/0.01/0.01, dsn=2.1.5, status=deliverable (250 2.1.5 OK)
Dovecot 2.2.7:
Nov 5 05:24:04 spectre postfix/qmgr[627]: 3dDHnc5J06z1s: from=double-bounce@spectre.leuxner.net, size=271, nrcpt=1 (queue active) Nov 5 05:24:04 spectre postfix/smtpd[2484]: 3dDHnc5KWcz1w: client=web.heise.de[193.99.144.71] Nov 5 05:24:04 spectre postfix/cleanup[2489]: 3dDHnc5KWcz1w: message-id=E1VdXxN-0002Gx-0s.01@web.heise.de Nov 5 05:24:04 spectre postfix/lmtp[2491]: 3dDHnc5J06z1s: to=tlx@leuxner.net, orig_to=newsletters@leuxner.net, relay=spectre.leuxner.net[private/dovecot-lmtp], delay=0.03, delays=0/0.01/0.01/0, dsn=4.4.2, status=undeliverable (lost connection with spectre.leuxner.net[private/dovecot-lmtp] while sending MAIL FROM)
While it still seems to work, mail gets delivered, it looks pretty ugly in the logs.
Regards Thomas