Up prioritized messageId and added documentation for it

e-invoice-domain/src/main/kotlin/net/codinux/invoicing/email/EmailsFetcher.kt

@@ -157,10 +157,10 @@ open class EmailsFetcher(
         val plainTextBody = getPlainTextBody(messageBodyParts, status)
 
         val email = Email(
+            status.folder.getUID(message),
             sender, message.subject ?: "", map(message.sentDate ?: message.receivedDate),
             message.getRecipients(Message.RecipientType.TO).orEmpty().map { map(it) }, message.getRecipients(Message.RecipientType.CC).orEmpty().map { map(it) }, message.getRecipients(Message.RecipientType.BCC).orEmpty().map { map(it) },
             (message.replyTo?.firstOrNull() as? InternetAddress)?.let { if (it.address != sender?.address) map(it) else null }, // only set replyTo if it differs from sender
-            status.folder.getUID(message),
             parts.any { it.mediaType == "application/pgp-encrypted" },
             imapMessage?.contentLanguage?.firstOrNull(),
             plainTextBody, getHtmlBody(messageBodyParts, status, plainTextBody),

e-invoice-domain/src/main/kotlin/net/codinux/invoicing/email/model/Email.kt

@@ -4,18 +4,31 @@ import java.time.Instant
 import java.time.ZoneId
 
 class Email(
+    /**
+     * Unique identifier of the message, used as identifier to retrieve further data of this message or as identifier
+     * of the last downloaded message (to continue fetching emails at messages newer than the message with this id).
+     *
+     * Actually the IMAP UID, but the user should not care which ID this value refers to.
+     * (messageUID: Unique identifier of a message in an email account. Survives e.g. expunging and moving the message to a different folder. This value.
+     * messageNumber: Non stable number. Message numbers e.g. change on expunge.
+     * messageId: Long string that is reference in inReplyTo field to identify to which message this message is a response of. Not of interest for us.
+     */
+    val messageId: Long,
+
     val sender: EmailAddress?,
     val subject: String,
     val date: Instant,
+
     val to: List<EmailAddress>,
     val cc: List<EmailAddress> = emptyList(),
     val bcc: List<EmailAddress> = emptyList(),
     val replayTo: EmailAddress? = null,
-    val messageId: Long,
+
     val isEncrypted: Boolean = false,
     val contentLanguage: String? = null,
     val plainTextBody: String? = null,
     val htmlBody: String? = null,
+
     val attachments: List<EmailAttachment> = emptyList()
 ) {
     val plainTextOrHtmlBody: String? by lazy { plainTextBody ?: htmlBody }