Somehow it lost another post I just did where I was saying that StreamWriter constructor has Encoding parameter. You can tell it to use any encoding (as long as appropriate codepages are installed on the system if the encoding isn't supported out of the box by .NET)

So you encode the text (from Unicode that mojoPortal uses internally I assume) to any encoding the user asked for and then you tell .NET to use TransferEncoding of base64 (to format the data for SMTP transfer since SMTP supports 7-bit, 1000 chars per line internally or something). Could use QuotedPrintable for single byte ones like ISO-8859-7 (not 7-bit that would probably zero the 8th bit), but base64 covers multibyte ones too like classic Unicode (UTF-16) or UTF-32 so it's better to use that for any non us-ascii encoding (including Greek, Russian etc.)

Strange, it says "If you set the BodyEncoding property to UTF8, Unicode, or UTF32, the Framework selects a TransferEncoding of Base64 for this MailMessage.", but only AttachmentBase has way to set TransferEncoding (btw, there's also a SubjectEncoding property for MailMessage you have to take in mind).

I guess they have some internal property to set the TransferEncoding for the MailMessage (maybe a protected one that you could surface using a descendent class [if allows inheritance]), else the doc is wrong or misleading there.

I think I know what they do though, they use MIME/Multipart and make an empty main message and add an alternate view as attachment to the AlternateViews collection (the attachment allows you to use TransferEncoding). AlternateViews should also be used with HTML mails I guess.

Use the AlternateViews property to specify copies of an e-mail message in different formats. For example, if you send a message in HTML, you might also want to provide a plain text version in case some of the recipients use e-mail readers that cannot display HTML content.

To add an alternate view to a MailMessage object, create an Attachment for the view, and then add it to the collection returned by AlternateViews. Use the Body property to specify the text version and use the AlternateViews collection to specify views with other MIME types. Use the MediaTypeNames class members to specify the MIME type for the alternate view.

Does mojoPortal support HTML mails that have fallback plain text currently? (as AlternateViews [MIME/Multipart])