We broadcast an HTML based email every two months.
Most everyone these days has their mail program set to allow html - there is a very small percentage that does not.
I would not recommend using flash, we did once and had a large percentage of the emails returned to us due to security settings.
Also, follow legislation
Federal E-mail Legislation - What You Need to Know
This month, the Federal Can Spam Act went into effect. Its goal is to stop the fraudulent practices associated with Unsolicited Commercial E-mail (UCE), not to hinder the operation of legitimate e-mail marketers. But, it is important for marketers to understand and abide by the law because these best practices are no longer optional. Here are some things you should know about the federal legislation.
Prohibits deceptive practices. As a legitimate e-mail marketer, these prohibitions shouldn’t impact your marketing efforts. The following fraudulent practices have been outlawed:
Accessing a protected computer without authorization to transmit multiple commercial e-mail messages
Using a protected computer to transmit messages with the intent to deceive to mislead recipients
False or misleading header information
Falsified identity
E-mail address harvesting and random address generation
Unsubscribe and Valid Reply. A “functioning return electronic mail address or other Internet-based mechanism” must be included in each message. In other words, you must provide an online method for recipients to opt-out, either through a return e-mail or a click-through link. Socketware suggests that you monitor all incoming messages for unsubscribe requests, regardless of the opt-out method you choose to offer.
The unsubscribe method must be functional for 30 days after you send the message and unsubscribe requests must be honored within 10 days. Be sure that recipients are unsubscribed from ALL of your lists or allow them to choose which communications they want to receive.
Valid Physical Address. The postal address of the sender must be included in all unsolicited e-mails. Socketware recommends that permission emailers include a physical address in their campaigns.
Deceptive Subject Headings. It is illegal to use a subject heading that would mislead a recipient about the contents or subject matter of the message.
Labeling. Messages that are opt-in do not require labeling unless they contain sexually oriented content. In upcoming months, the Attorney General will prescribe the labeling to be included with sexually explicit messages.
State Pre-emption. The Can Spam Act overrides existing state laws, even those that are stricter than the federal legislation.
“Do Not E-mail” List. The Do Not E-mail list has not been signed into the law yet. The Can Spam Act gives the Federal Trade Commission (FTC) permission to study the possibility of the list and suggest ways to implement it. The FTC will report to Congress in six months with their findings.
TD