Email html rendering/base64?

I'm absolutely mystified. I'm trying to send an html email which looks fine in loads of webmail browsers....but....someone with an IPad which has the apple native email client. says it renders as raw html, as it does in thunderbird.
When I looked at the source the content-transfer-encoding : base64, but an email which looks ok in the applemail is QuotedPrintable
My c# code is:
                mail.BodyEncoding = System.Text.Encoding.UTF8;
                mail.BodyTransferEncoding = System.Net.Mime.TransferEncoding.EightBit ; (I've tried SevenBit+QuotedPrintable)
                SmtpServer.DeliveryFormat = SmtpDeliveryFormat.SevenBit;

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I've tried a coupla others, but the content-transfer-encoding is always turning up as base64?
I can't believe I've just come across this and there's not much on the web about it.
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ste5anSenior DeveloperCommented:
I don't think it's an encoding problem. What does "raw html" mean? Have you considered the fact, that HTML e-mail bodies can only use a HTML subset and that different e-mail clients render this differently?

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Silas2Author Commented:
I mean the email app displays the words
<html><body> etc
, rather than rendering it.
When I look at the source (I mean the whole source including headers) the only difference I can spot between an email which renders ok in the apple app/thunderbird and my one is the encoding, the successful apple one is visible in plain text, my one is base64/garbled in view source, that's why I think its the may not be.
What content-type are you setting? Some API provide a method like setHtml() and internally adjust the content-type accordingly. Try the "text/html"  content type.
Silas2Author Commented:
This is so spooky, I've tried every combination...but I have to confess I have a .net mail at another website which is using the defaults (i.e. setting no values for contenttype/transferencoding) and its working fine.
For some reason this one keeps sending utf8/base64 even if I force ascii to contenttype.
I've just seen this very old (2005) post about

With System.Net.Mail we look at the body and kind of determine what the
best encoding to use. If your body has non ascii characters you should not
set the encoding as ASCII.
so maybe some dirt in the message is forcing the base64 encoding? (I can't see any...)
Silas2Author Commented:
Ah , you know what it is, AlternativeView, I was using one of those, that seems to force  base64.
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