Tried to run imaptest stress testing against maddy.
Using go-imap 1.0.0.beta.4 and go-imap-sql (dev branch).

Command:

imaptest seed=666 port=9999 copybox=INBOX2 user=testuser pass=foo mbox=~/Downloads/firefox/dovecot-crlf

Mentioned mbox file is here: http://www.dovecot.org/tmp/dovecot-crlf.

Outputer after running is two seconds: imaptest_stress.log (look for "Error:" lines)

Looks like there is some issue that makes responses get intermixed with literals in responses.
Adding cilents=1 leaves only

Error: testuser[300]: DELETE failed: 300.6 NO Flags must be a list

Need to check against RFC if this is a valid syntax.

*** Test close command 1/4 (line 3)
 - failed: Expected tagged reply 'ok', got 'NO Flags must be a list'
 - Command (tag 4.13): store 1,3 +flags \deleted

Relevant ABNF from RFC 3501.

flag            = "\Answered" / "\Flagged" / "\Deleted" /
                  "\Seen" / "\Draft" / flag-keyword / flag-extension
store           = "STORE" SP sequence-set SP store-att-flags
store-att-flags = (["+" / "-"] "FLAGS" [".SILENT"]) SP
                  (flag-list / (flag *(SP flag)))

Turns out STORE * +FLAGS $FlagA $FlagB is also valid and equivalent to STORE * +FLAGS ($FlagA $FlagB).

Adding to the v2 milestone. There is number of issues detected by imaptest that could be resolved only using breaking changes (see #323).