www.law.cornell.edu/uscode/text/18/2258A

(a) Duty To Report.—
(1) In general.—
(A) Duty.—In order to reduce the proliferation of online child sexual exploitation and to prevent the online sexual exploitation of children, a provider—
(i) shall, as soon as reasonably possible after obtaining actual knowledge of any facts or circumstances described in paragraph (2)(A), take the actions described in subparagraph (B); and
(ii) may, after obtaining actual knowledge of any facts or circumstances described in paragraph (2)(B), take the actions described in subparagraph (B).
(B) Actions described.—The actions described in this subparagraph are—
(i) providing to the CyberTipline of NCMEC, or any successor to the CyberTipline operated by NCMEC, the mailing address, telephone number, facsimile number, electronic mailing address of, and individual point of contact for, such provider; and
(ii) making a report of such facts or circumstances to the CyberTipline, or any successor to the CyberTipline operated by NCMEC.

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(e) Failure To Report.—A provider that knowingly and willfully fails to make a report required under subsection (a)(1) shall be fined—
(1) in the case of an initial knowing and willful failure to make a report, not more than $150,000; and
(2) in the case of any second or subsequent knowing and willful failure to make a report, not more than $300,000.

Check with a lawyer if blocking an upload that your server has access to because of suspected CSAM constitutes “actual knowledge or any facts or circumstances”.