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Is it Possible to Reply From an .EML file?

I would like to know if it is possible to open and reply as a response message on top of a saved .eml file using C#?
If yes, are there any guides that I can follow?
I am able to do so with EWS EmailMessage where the message resides in Inbox. However, due to requirements, I am required to retrieve and reply from a saved .eml file instead of replying from the email in Inbox directly.
Thank you.

No you can't do this using the EWS reply operations because EWS is a Server side API while using the EWS Managed API you using Client side library that just instrumenting the SOAP call to the backend whenever an action is taken. You can temporary import the EML back in the server https://learn.microsoft.com/en-us/exchange/client-developer/exchange-web-services/how-to-import-items-by-using-ews-in-exchange and then you reply and then delete the Message again if don't want it to exist. If your want to send using EWS you didn't want to import the message use something like mimekit to generate the MIME of the response message offline http://www.mimekit.net/docs/html/Frequently-Asked-Questions.htm then you can just sent the Mime generated by mimekit via EWS.

In theory you could achieve this, by first opening the EML file (please see this question and answer):
Retrieve Email Information from .EML Files
And then when you have the contents of the message in code, copy the data over to whatever email-sending API you are using.

Step 1:-
Click the message options menu in the top right corner of the message. Select Save email.
Step 2:-
Save the file. The file name will default to the subject line of the email, with special characters removed. Your download options depend on whether you're using Front on the web or in the desktop app:
*Web: Your file automatically saves to wherever you have it set up in your browser settings (like your Downloads folder)
*Desktop app: You will get the “Save As” prompt where you can rename the file and choose your download location
Step 3:-
You will see your .eml file in your chosen location. You can attach it to a message via drag-and-drop or with the attachment file picker.

Is it possible to send an attachment in Outlook as a URL

I have an Outlook addin that handles attachments by sending them to our server and then embedding a link into the body of the email containing the URL to the uploaded attachment.
This works but it is not 100% satisfactory because the URLs can get broken depending in the email client and the sender will not see their attachments listed at the top of there email where they are use to seeing them.
Ideally what I would like to do is exactly what is being done for OneDrive.
Currently if I add an attachment and provide a URL as the source it will download the file and send it as a normal attachment which is the exact opposite of what I want. I have noticed that if it fails to download the file it will then just send the URL as an attachment which is what I want. But sending broken URLs is not very useful.
Using
Outlook.MailItem.Attachments.Add("https://myserver.com/somefile", 7, 1, "MyAttachment")
If I set the attachment type to '7' which is what is used for OneDrive attachments I can get it to send the URL but it will first complain about not being able to set some access rights and if the sender clicks OK to continue anyway the attachment will appear to the sender and the receiver as residing on OneDrive which it is not, but clicking the URL will download the file from my server as I want.
My question is: Is it possible to send a none OneDrive URL as an attachment?
(I expect the answer is no, but I am hoping some Outlook guru out there may know some trick to do this.)
My other alternative it to wrap my URLs in an small html file that is then sent as a normal attachment and when opened will provide the link to the actual file.

attachments with ics file in c#

I am trying to send an ICS (calendar) file to someone that read it using Office365, using C#.
Sending just the ICS works fine. But I have to attach a file.
At this point, the mail is received without calendar, or the calendar is attached to the email and cannot be used, or the attachment is missing.
I tried 2 different ways:
1. send attachment inside VEVENT, using ATTACH;ENCODING=BASE64;VALUE=BINARY;X-FILENAME=file.txt:, but this does not show any attachment in Office365 (like here: Attaching a file to an iCalendar)
2. send attachment as a separate file, and attach it to VEVENT using ATTACH:CID:, but this time the calendar file is displayed as attachment
The second way should work if I can format the mail message to look like this:
multipart/mixed
multipart/alternative
text/plain
text/calendar;method=REQUEST
text/calendar (with a content-disposition:attachment)
(as described in Multipart email with text and calendar: Outlook doesn't recognize ics)
If I send a calendar invite using Outlook365, the message looks like that.(but instead of the calendar as attachment, the image or doc is attached)
but using System.Net.Mail.MailMessage, I can generate a message like this
multipart/mixed
multipart/alternative
text/plain
text/calendar;method=REQUEST
multipart/mixed
text/plain (with a content-disposition:attachment)
Desktop version of Outlook 2007 shows both calendar and attachment, so it seems that Outlook365 is very strict about the format.
But how can I send, using .NET classes, an email that does not have the extra multipart/mixed section? MailMessage class generate for the Attachments collection a new section with content type multipart/mixed, insead of adding them to the top level.

Outlook Add-In - Custom Headers for Email Tracking

We have written an Outlook add-in in C#, that appends a custom header to outgoing messages.
This add-in has to use a library called Redemption to bypass Outlook's security to modify the headers, and this is all working great.
Our problem lies when sending outgoing mail through an exchange server. We use the additional header as such:
Add a References header with an email address that includes an ID for tracking with our system.
This is a standard email header that all mail clients should pass on when replying to messages. So replying to a message will automatically keep the new message tracked.
All of this works just fine if you send an email from an IMAP account setup in Outlook such as GMail.
Problem is, if you send mail via an Exchange account, the Exchange server overwrites the References header and uses it's own proprietary headers: Thread-Index and Thread-Topic. Email standards suggest to use References and In-Reply-To headers. See this link on this issue.
Does anyone know a way around this? Some algorithm to gain us the following:
Add a header (of any name or kind) to emails that includes a 10-digit ID and 3 letter prefix
Replying to this email from all (or most) mail clients preserves the custom header

I think the following algorithm is going to solve our issue:
Our Outlook add-in will set the References and an arbitrary X- header
Our mail filter will look for References, if found use it (if outgoing mail server was Exchange, it will not be present)
If our mail filter finds the arbitrary X- header and a Thread-Index, it will store the data found in the X- header.
Later if the email is replied-to several times, the mail filter will use the Thread-Index to look up the past info to keep the email tracked.
For those wanting to know internals of the Thread-Index header, it is a Base64 encoded string. The first 22 bytes are the original unique portion and each reply adds an additional 5 bytes on to it. We only use the first 22 bytes to identify the email.

Force mail client to use text rather then HTML through mailto:

I'm using SubVersion and TRAC on a C# project I am working on, and I have my TRAC system setup with a email address that can be used to create tickets. In my program I've added a simple "FeedBack" button in my program which sends an email to this address. To open the email I'm just "starting" a mailto link as shown below.
System.Reflection.Assembly assem = System.Reflection.Assembly.GetExecutingAssembly();
string ver = assem.GetName().Version.ToString();
System.Diagnostics.Process.Start("mailto:foo#bar.com?subject=<Provide a title for your feedback here>&body=< Describe the problem you are having or enhancement you would like to suggest here. Please be as descriptive as you can, and if possible list out the actions that will replicate the problem >%0D%0A%0D%0A%0D%0AVersion: "+ver);
The problem I'm running into is if the user is using Outlook and their copy of Outlook is setup to HTML the ticket that gets created ends up having a bunch of HTML code that I have to clean up. Is there some way to notify whatever mail client is handling it to send the email as text rather then HTML?

There's nothing you can do (besides education) on the client - there's nothing in mailto to control a client side program. And, frankly, with the proliferation of web-based email - I think mailto is showing it's age.
Outlook should send a mime/multipart message, with both plain text and HTML parts. I'd guess you could extend or patch Trac to only grab the text/plain portion.
Otherwise, just create a form in your app to capture the email info. Again, if someone is using Hotmail or GMail - mailto is not likely to work anyway (or will open up their unconfigured Outlook Express, where they will dutifully type up an email and press Send. Only it won't go anywhere, because no SMTP servers are configured - so it will languish in the Outbox for years. Not that they will notice though...).