SASL stands for "Simple Authentication and Security Layer". SASL itself is nothing more than a list of requirements for authentication mechanisms and protocols to be SASL-compatible as described in RFC 4422. IMAP, POP3 and SMTP protocols all have support for SASL.
Many people confuse SASL with one specific SASL implementation: the Cyrus SASL library. Dovecot has its own SASL implementation which may at some point be separated from Dovecot itself to "compete" against Cyrus SASL library in server side.
Dovecot SASL can already be used with:
Postfix v2.3 and later. See HowTo/PostfixAndDovecotSASL for details. Exim v4.64 and later. See HowTo/EximAndDovecotSASL for details.
SASL (Simple Authentication Security Layer) is an Internet standards-track method for remote computers to authenticate. The Cyrus SASL library makes supporting various SASL mechanisms easy for both client and server writers.
The Cyrus project is the implementation of an enterprise mail system by the Carnegie Mellon University Computing Services Department. We are interested in scalable, easy to administer systems.
从上面的描述中能很清楚的看出，SASL其实是一个定义，且全称是"Simple Authentication and Security Layer"，而Cyrus SASL是SASL的一种具体实现的套件，而不是开始理解混淆的SASL就是实现SMTP验证的一个套件。Postfix+Dovecot方案中之所以可以不用到Cyrus SASL这个套件是因为Dovecot套件中也有自己SASL的具体实现，Dovecot并不只是一个IMAP+POP3的实现。