libqmail is a library of general purpose APIs extracted from Dan Bernstein's software, reimplemented and covered by the GNU General Public License Version 2.
"When using the SASL SCRAM mechanism, the SCRAM-SHA-256-PLUS variant SHOULD be preferred over the SCRAM-SHA-256 variant, and SHA-256 variants [RFC7677] SHOULD be preferred over SHA-1 variants [RFC5802]".