This is one of usual guidelines that is being followed in all most of the groups. You have to send the mail to a group as a text message and not as html. This is done to avoid the mails getting detected as spam. Some of the mails are getting detected as spam these days.
For Gmail Users.
If you are gmail user, just above the composing space you can see a link like this Plain Text Formatting at the right side or a Rich formatting at the left side. If you could see Plain Text formatting, then you are sending the mails as html, so click that link to send the mails as text. If you can see, Rich formatting, then you are sending the mail properly.
For Yahoo Users:
Go to options at the top right corner, you can see various options. Select General preferences. There you could see another option "Composing Emails". Select "Compose messages as plain text". Save and start using the account.