The latest bogus AmEx email to hit inboxes is a warning about an aborted transaction / terminated incoming transfer:
Unfortunately, all of the links included in the email will take the worried user to one of many compromised websites hosting the Blackhole exploit kit.
The kit then exploits an Adobe Reader and Acrobat vulnerability to drop an information-stealing Trojan onto the victim's machine.
According to Webroot, when this malicious campaign was first spotted and analyzed more than a week ago, the malware in question was detected only by two of the 43 AV solutions used by VirusTotal. Since then that number has reached 15, but that is still a very poor detection rate.
If you have been duped into clicking on the links you should consider seeing which AV solutions detect the malware and use one of them to check your machine.