Flags are converted to RawString without any check

[emersion]

If the user calls Client.Append with flags containing invalid chars (e.g. a space), go-imap will send garbage to the server. We should check such user-provided strings.

Maybe we can introduce a function like func FormatRawString(s string) (RawString, error).

[Adirelle]

Note that the keyword flags are a special case, which may require a different validation than quoted strings.

[emersion]

Fixed in v2.