In this case, I think you could use either "said" or "says." You would have to use "said" if you were talking about an event that happened entirely in the past. For instance, "Last month, I read the email that said the meeting would be postponed. The meeting was held a few days later." You use "says" if you're talking about something that is still ongoing. "I just got an email from my boss. The email says we all need to update our passwords this week." However, it would not be incorrect to use "said" in this case, since you're talking about an email that you read in the past. The only time you have to use the present tense "says" is if you're talking about an email that you're currently reading. Because you're talking about an event that is ongoing ("the meeting will be postponed" or "there was a mistake in the schedule" -- assuming this is a current schedule that's relevant in the present), it's okay to use "says." But it's also okay to use "said," because you read the email in the past. If your main focus is on an ongoing problem that you're complaining about, it might sound best to use "says."

I read the email SAYING that the meeting...

I read the email that said that the meeting will be postponed/ that there was a mistake in the schedule

A bit more context would be helpful I think since what you say next in the sentence will determine which one to use.