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If you want to keep all 25 and you're fully utilizing those segments, you can request for Marketo to increase your segmentation limit. You will have to submit a business case on why you need more segments. The caveat to that is there's a chance that it might slow down your instance.
Does it give you any error message? I've receive error messages before or have tried to send it to a different email address. Sometimes, it was the content within the email like a broken token or script.