System.Net.Mail creating invalid emails and eml files? Inserting extra dots in host names

This is actually per RFC 2821 (4.5.2 Transparency)

Before sending a line of mail text, the SMTP client checks the first character of the line. If it is a period, one additional period is inserted at the beginning of the line.

.Net is just storing the file in "ready to transmit" mode which means that it doesn't have to monkey with the email before sending, instead it can transmit it as is. Unfortunately this format isn't 100% the same as Outlook Express's EML format apparently. You should be able to set the encoding to UTF-8 (or something similar) and that will kick in Base-64 encoding for you.

mail.BodyEncoding = System.Text.Encoding.UTF8;

In .Net 2.0

X-Sender: [email protected]
X-Receiver: [email protected]
MIME-Version: 1.0
From: [email protected]
To: [email protected]
Date: 6 Jul 2011 21:29:04 +0100
Subject: Test
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable

Hello this is  a short test of the issue: <a href=3D'https://test.com/'>https://test.com/</a>:=

It looks like it is wrapping the text at a certain character length per line. I vaguely remember there was an issue in .Net 2.0 where by default it doesn't do this which can cause problems with spam filters.

In fact increasing the size of the message gives the following in .Net 4.0:

X-Sender: [email protected]
X-Receiver: [email protected]
MIME-Version: 1.0
From: [email protected]
To: [email protected]
Date: 6 Jul 2011 21:34:21 +0100
Subject: Test
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable

Hello this is  a short test of the sssssssssssssssssissue: <a hre=
f=3D'https://test.com/'>https://test.com/</a>:=20

Seems like a bug.

A workaround might be to change the BodyEncoding to something other than ASCII.

Looking at .NET 4 source code, maybe what you are experiencing has something to do with MailWriter.WriteAndFold method. Also in MailWriter class there is

static int writerDefaultLineLength = 76

variable, meaning character count per line. Maybe because you removed one extra space character, it started working.