\* is not a valid IMAP flag

[nevans]

The default FLAGS response for the SELECT command sends \*, which is valid for PERMANENTFLAGS but not for FLAGS. See RFC3501 or RFC9051:

The PERMANENTFLAGS list can also include the special flag \*, which indicates that it is possible to create new keywords by attempting to store those flags in the mailbox.

(emphasis is mine)

The relevant formal syntax (note that * is not a valid ATOM-CHAR):

atom            = 1*ATOM-CHAR

ATOM-CHAR       = <any CHAR except atom-specials>

atom-specials   = "(" / ")" / "{" / SP / CTL / list-wildcards /
                  quoted-specials / resp-specials

list-wildcards  = "%" / "*"

flag            = "\Answered" / "\Flagged" / "\Deleted" /
                  "\Seen" / "\Draft" / flag-keyword / flag-extension
                    ; Does not include "\Recent"

flag-extension  = "\" atom
                    ; Future expansion.  Client implementations
                    ; MUST accept flag-extension flags.  Server
                    ; implementations MUST NOT generate
                    ; flag-extension flags except as defined by
                    ; future standard or standards-track
                    ; revisions of this specification.

flag-keyword    = atom

flag-list       = "(" [flag *(SP flag)] ")"

flag-perm       = flag / "\*"

mailbox-data    =/ "FLAGS" SP flag-list

resp-text-code  =/ "PERMANENTFLAGS" SP "("
                  [flag-perm *(SP flag-perm)] ")"

And the relevant code is here (I think):

greenmail/greenmail-core/src/main/java/com/icegreen/greenmail/imap/commands/SelectCommand.java

Line 50 in 2aa14fc

response.flagsResponse(mailbox.getPermanentFlags());

Perhaps a mailbox.getFlags() method could be added that simply filters out \* from mailbox.getPermanentFlags()?

[nevans]

By the way, do you know if any "real" servers respond with \* as a (non-permanent) flag? I ask because I haven't seen any myself and I'd prefer to keep my client protocol parsing code strict on this. But if it does occur in the wild, I'll pragmatically parse more loosely.

[nevans]

oops... I accidentally tapped the close button there!

[marcelmay]

Thx alot for the detailed bug report, @nevans .

Regarding you question: I can not say anything about 'real' mail servers, unfortunately.

[marcelmay]

Interesting: The SELECT FLAGS result (availableFlags) is more or less ignored in JakartaMail reference implementation ...