EwsId reverse engineered format

EwsId_format.txt

IdStorageType (byte):
    MailboxItemSmtpAddressBased = 0, // Exchange 2007 (EwsLegacyId)
    PublicFolder = 1,
    PublicFolderItem = 2,
    MailboxItemMailboxGuidBased = 3,
    ConversationIdMailboxGuidBased = 4,
    ActiveDirectoryObject = 5, // Exchange 2013
    MailboxItemMailboxGuidBasedWithFallback = 6 // Exchange 2016

CompressionId (byte):
    NoCompression = 0,
    RleCompression = 1

IdProcessingInstruction (byte):
    Normal = 0,
    Recurrence = 1,
    Series = 2,
    Multiple = 3
    
RLE compression:
    If two bytes following each other are distinct then the first byte is verbatim
    If they are equal then the byte following contains count - 2,
        i.e. if buf[i] == buf[i+1] then output buf[i] c+2 times where c = buf[i+2].
        (or output buf[i] and buf[i+1] are printed verbatim, then buf[i] is printed
            buf[i+2] more times and buf[i+2] is then skipped)
    
Base64 encoded EwsId:
0       CompressionId               compression id
1-...   <payload (max length 1024)>

Decompressed payload:
0       IdStorageType               storage type
1-...   <storage specific content>

CountedContent:
0       Int16                       len
1-len   <content>

DateTime: .NET datetime, 2 bits Kind, 62 bits 100-nanosecond ticks since 01-01-01 0:00:00 UTC

Moniker: CountedContent
Email moniker: Moniker with UTF8 encoded string, max length 1000
Guid moniker: Moniker with Guid as "D" format ascii string, max length 50

InternetMessageId: CountedContent with UTF-8 string
StoreId: CountedContent, dependent on IdProcessingInstruction
    case Multiple:
        CountedContent+
        (Conversion to StoreObjectId uses first content as ENTRYID)
    case Normal, Series:
        ENTRYID
    default: (Recurrence or unknown)
        if last byte == 16:
            OccurenceStoreObjectId
        else:
            ENTRYID
    
OccurenceStoreObjectId:
0       byte                        value: 8
1-8     DateTime big endian         OccurenceDate
9       ENTRYID

MailboxItemSmtpAddressBased storage content:
        Email moniker               MailboxId
        IdProcessingInstruction     IdProcessingInstruction
        StoreId                     StoreId

MailboxItemMailboxGuidBased, ConversationIdMailboxGuidBased storage content:
        Guid moniker                MailboxId
        IdProcessingInstruction     IdProcessingInstruction
        StoreId                     StoreId

MailboxItemMailboxGuidBasedWithFallback storage content:
        Guid moniker                MailboxId
        IdProcessingInstruction     IdProcessingInstruction
        StoreId                     StoreId
        InternetMessageId           InternetMessageId
        
PublicFolder storage content:
        StoreId                     StoreId

ActiveDirectoryObject:
        CountedContent<Guid>        ADObjectId
        
PublicFolderItem storage content:
        IdProcessingInstruction     IdProcessingInstruction
        StoreId                     StoreId
        StoreId                     FolderId
        
Id types:
    EntryId.length == 46: FolderId
    EntryId.length == 70: MessageId
    (EWS validates that the EntryId has one of these length)