I tried this with mail sent from Thunderbird and didn’t have a problem.
How certain are you that the change is happening in the Domino mail router? If you select “keep sender’s format,” I would expect the MIME contents to be simply plopped into the Body item with no changes – it’s HTML we’re talking about – the client renders it. Making changes to the contents is none of the server’s business (except for virus checkers and spam filters – but I can’t see them changing links in this way).
I wonder whether you could send the message to some other mail server and make sure the change isn’t caused by the sending mail client. A lot of them make rather free with text the user edits.
Check the value of the field “Format preference for incoming mail:” within your Person Document. Here’s a list of the possible-values: a. Prefers MIME
b. Prefers Notes Rich Text Format (RTF)
c. Keep in sender’s format
Options A or B could induce a conversion depending on where the message was sent-from (i.e. Notes client or Internet), whereas “Keep in sender’s format” should allow the message’s contents to remain “as-is”.
URL encoding changes most special characters to the text %xx where xx is a hexadecimal number representing the value of the character. In particular, spaces are changed to %20.
A backslash () is changed to a forward slash (/) rather than encoded. Double backslashes (\) are removed. Dashes (-) are passed through as is.
Based on-that, avoid using the backslash in a URL as it isn’t a reserved special-character. Instead, use the hexidecimal value for the backslash: