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Logic changes in TIdMessage regarding Message-ID and In-Reply-To headers

For a long time, TIdMessage would generate a "Message-ID" header if its MsgId property was not blank, but only when saving an email to a file, not when streamed or transmitted.  Otherwise the MsgId was ignored and "Message-ID" was omitted.

However, many modern email servers will reject an outgoing email as spam if it does not have a client-generated "Message-ID" header, especially when sending a new email versus replying to another email.  RFCs encourage clients to generate their own "Message-ID", and many popular email clients (Outlook, Thunderbird, etc) do exactly that.  And, Indy users have been asking for this ability.  So, TIdMessage has been updated to always generate a "Message-ID" header if the MsgId property is not blank, regardless of whether the email is being saved, streamed, or transmitted.  In a future release, TIdMessage may be updated further to auto-generate a "Message-ID" value if the MsgId property is blank.

Just know that if you reuse a TIdMessage object to create a reply email from another email, be sure to assign a new value to the MsgId property after copying the old value to the InReplyTo property, so else the reply will have the same "Message-ID" as the email being replied to.

Speaking of "In-Reply-To", a logic change has been made for that header as well.  For a long time, TIdMessage would always generate an "In-Reply-To" header if the InReplyTo property was blank but a "Message-ID" header was present (either from the MsgId or ExtraHeaders property), where the "Message-ID" value would be copied to the "In-Reply-To" header.  However, when sending a new email, this confuses email servers, and can cause rejections as well.  "In-Reply-To" should only be used in reply emails.  So, TIdMessage has been updated to no longer generate an "In-Reply-To" header if the InReplyTo property is blank.  When sending a reply email, you are now responsible for populating the InReplyTo property as needed.


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