sendmail, 4th Edition

AuthMaxBits

Limit max encryption strength for SASL V8.12 and later

When a client’s site connects to the server, the server can offer authentication by presenting the AUTH keyword, followed by authentication mechanisms supported:

250-host.domain Hello some.domain, pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-AUTH DIGEST-MD5 CRAM-MD5 KERBEROS-V4             ← note this line
250-DELIVERBY
250 HELP

If the connecting site wishes to authenticate itself, it replies with an AUTH command indicating the desired mechanism:

AUTH CRAM-MD5
← authentication challenge here
← authentication reply here
235 Authentication successful.           ← server replies

This interaction automatically establishes an authenticated stream using the CRAM-MD5 method.

If you wish to turn off additional encryption in SASL when STARTTLS is already encrypting the communication, you do so by defining this AuthMaxBits option. When set, this option limits the maximum encryption strength for the security layer in SMTP AUTH. When not set (the default), encryption strength is essentially unlimited. The AuthMaxBits option is used like this:

O AuthMaxBits=limit                     ← configuration file (V8.12 and later)
-OAuthMaxBits=limit                     ← command line (V8.12 and later)
define(`confAUTH_MAX_BITS', `limit')    ← mc configuration (V8.12 and later)

Here, limit is the maximum number of bits in the key length. The existing encryption strength is taken into account when choosing an algorithm for the security layer. For example, ...