.NET and SMTP should support any encoding, as long as you use base64 for the TransferEncoding (for single-byte ones you could also use QuotedPrintable [instead of 7-bit which will eat up data for sure], but using base64 is best since it also support multibyte ones like UTF-16 [the classic Unicode encoding]).

At LvS, at LvS_Utilities\subtitles\BaseSubtitleWriter.vb, I'm using:

Using writer As New StreamWriter(path, False, theEncoding)

  WriteSubtitles(subtitles, writer)

End Using

so you could use StreamWriter to write to any encoding the user told you to use, then tell .NET to use base64 for the TransferEncoding so that SMTP can transfer it fine

My experience on the above is from lots of research I had to do recently to fix osCommerce (well known PHP store) to send e-mails with Greek text. BTW, in osCommerce they have a store name that is used as the display name for e-mails (decent e-mail clients show both the display name AND the e-mail address, so there's no issue with phising - anyway SMTP headers can be faked to say anything - my comment regarding spam filters was that I think they have higher chance to catch a post that just has a newly seen from e-mail address with no display name, but I might be wrong with that [could check SpamAssassin's rulebase for example, although other engines use PETRI nets, neural nets etc. that are not always easy to checkout])

Also, regarding message body, .NET supports any encoding from what I understand, according to what is written at reference of BodyEncoding class on MSDN, not just us-ascii and "Unicode" (in the general sense) ones, like UTF-8, UTF-16 (this is called Unicode encoding officially in .NET if I'm not mistaken) and UTF-32. It's just that for the last 3 ones .NET selects a TransferEncoding of Base64 (instead of 7-bit or QuotedPrintable) for the MailMessage. 

From your replies above I understood you were saying .NET doesn't support say Russian encoding (or ISO-8859-7 [Greek]) etc. which doesn't sound accurate (in fact some older mail clients don't support UTF-8 so many admins would opt for using ISO-8859-7 in Greece)

The * is what is supported irrespective of codepages installed on the server. Such codepages are installed from Regional Settings (or if you have localized OS are preinstalled). You'd expect that an admin who wants to send ISO-8859-7 Greek text will have that codepage on their server, or ask their host (say if they use shared hosting) to add it (easy and no real overhead to do [just tiny disk space fraction to install the .NLS file]).