The rule is simple: you have to be wearing something. Full nudity will be rejected at the mail room at virtually every correctional facility in the country, and the entire envelope may be confiscated along with everything else inside it.
Lingerie and bikinis are fine. Photos showing suggestive poses in swimwear or underwear pass through without issue at most facilities as long as private areas are covered. That is where the line is, and staying on the right side of it means your photos actually reach your inmate.
The more covered you are, the less risk there is of any individual mail room officer making a judgment call against the photo. When in doubt, think swimsuit rather than less.
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