[Dovecot] LDA vs. LMTP

[Stan Hoeppner]

On 7/29/2013 6:05 PM, Ben Morrow wrote:
> At  4PM -0500 on 29/07/13 you (Stan Hoeppner) wrote:
>> On 7/29/2013 2:30 AM, Jan Behrend wrote:
>>
>>> You cannot use the LDA method if SMTP and IMAP services reside on
>>> different machines, which would be the case in larger scale mail system
>>> setups.
>>
>> Which brings up an interesting point.  With a single LMTP daemon on the
>> Dovecot server communicating via a single socket with the upstream MTA
>> over the wire, it would stand to reason that message throughput rate may
>> be limited by serialization in the LMTP request/reply chain.  There is
>> no parallelism, and thus there is relatively high latency.

You snipped the text where I stated this is a theoretical discussion,
due to the high msg volume required to prove one over the other.  That
said, I'll gladly continue to postulate on the theoretical.

> What makes you think an SMTP server delivering over LMTP only makes a
> single connection to the LMTP server? I believe Postfix by default makes
> a fresh connection for each delivery.

No, Postfix by default uses connection caching w/both SMTP and LMTP:
http://www.postfix.org/postconf.5.html#lmtp_cache_connection

If the load is sufficiently high it will open additional connections,
but it attempts to reuse existing connections as much as possible to
eliminate additional connection setup delays, which can be considerable
with SMTP servers.  For instance some OPs insert 2 minute or longer
greet delays as a (very crude) anti spam bot measure.  Connection
caching is an SMTP optimization, and not nearly as beneficial to LMTP.
The Postfix SMTP/LMTP clients are literally the same code.

-- 
Stan