At my work, we are using a lot of mails to send notifications to our customers.
We recently decided to switch every mails to full utf-8 compliant base64 encoded mails. That's the only way we can handle every languages around the world easily in 1 single format. Even tools that detects languages, like gmail, encoded messages in utf8/base64 when it detects multiple different languages in the same email.
That causes us some troubles first. Some antispam software automatically tags these mails as spam, as he have trouble decode them and analyse the content. So, to avoid that, we used a multipart email, with a text/html section, + a text/plain section. That solved our antispam trouble.
But, we discovered after that, that our customers using outlook 2003, and only 2003, not 2007, 2010, or even 2002 (XP) have this trouble.
What Outlook 2003 does, he look at the first part of the mail. He detects that the first part is the text/plain / ASCII / 7bit encoded part. He chooses us-ascii as encoding to display the mail. Then, it sees that the real mail is in the text/html second part. So, it displays the second part, but he choses the encoding of the first part. If I put my HTML part before the plain text, a lot of other emails clients are displaying the plain text section instead of the html section.
That's what gmail does. So, a good plain / html email must have the TEXT part first, followed by the HTML part. Every emails clients in the world understand this, except Outlook 2003.
So, just because of this, we'll have to add an option to our customers to remove the plaintext section of the mail if they are using Outlook 2003, with the consequence that the e-mails can be blocked by their antispam software.