Apostrophes turn into question marks via Posterous XML-RPC

Sending text email to blog@posterous.com results in a formatting error I believe is related to WordPress. All apostrophes come out as question marks via Posterous autopost.

Example:

This is the opening sentence of my blog entry, ‘blogentry’.

becomes:

This is the opening sentence of my blog entry, ?blogentry?.

The person sending the emails to the site is not me, it is a person for whom I built the site. We are working together across the Internet, and he does not have any computer knowledge to speak of, so I can’t really ask him if he has sent the email in rich text format, utf-8, etc. Also, I strongly prefer not to set up another account to post myself and test various methods, because there are a large number of RSS subscribers who will be pinged – rather unprofessional.

I have two other sites that I use many of the same plugins as this one. On both of those sites, there is no such issue. I also tested an MU site.

I tested both UTF-8 encoding and Western ISO and could not reproduce the error on the other sites.

Here is a list of plugins used on the site in question:

All in One SEO Pack
cbnet Ping Optimizer
Fast and Secure Contact Form
Free Traffic Getting SEO PluginTM 2.0
Google XML Sitemaps
Image Widget
IntenseDebate
Permalinks Moved Permanently
Sociable
Theme Tweaker
Ultimate Google Analytics
Widget Context
WordPress Database Backup
WP-Cumulus

Thanks in advance!