Opinion by
Judge MacPhail,Petitioner Effort Foundry, Inc. (Employer) appeals to this Court from an order of the Unemployment Compensation Board of Review (Board) reversing the order of the referee and granting benefits to Clyde H. Alternóse, Jr. (Claimant). The basis for the Board’s order was its conclusion that because Employer did not appeal from an initial grant of benefits to Claimant, the Bureau (now Office) of Employment Security (Bureau) was without jurisdiction to issue a revised determination denying benefits to Claimant. The only issue before us is whether, on the facts of this case, the failure of Employer to appeal from the initial grant of benefits to Claimant was justified and, accordingly, whether the Bureau’s re-determination was proper. We hold that it was. We reverse the order of the Board and remand this case for further proceedings on the issue of Claimant’s eligibility for benefits.
The facts of this case are crucial to our determination. Claimant was last employed by Employer as a grinder at a rate of $5.00 per hour. His last day of work was October 21, 1977. Claimant applied for unemployment compensation benefits and notice of his application was sent to Employer at “Effort Foundry, Effort, Pa. 18353,” the address supplied to the Bureau by Claimant. In fact, the correct zip code for *359Effort, Pa. is 18330. Employer had registered with the Bureau using the address of its office, P. O. Box 3167, Butztown, Pa. 18017. Employer’s letterhead stationery contained both addresses, the former denoted as “Plant” and the latter as “Office.” The “Plant” or Effort address was subordinate to the “Office” or Butztown address on the letterhead.
It is undisputed that Employer received notice of Claimant’s application for benefits, completed the employer’s statement thereon, and returned the statement to the Bureau. Employer did not correct the incorrect zip code. On November 28, 1977, the Bureau granted unemployment compensation benefits to Claimant. The Bureau’s records indicate that the Notice of Determination approving Claimant’s application was sent to Employer at “Effort Foundry, Effort, Pa. 18353.” The Notice of Determination stated that Employer had until December 13, 1977 to file an appeal from the determination. No appeal was filed.
Employer asserts that it never received the Notice of Determination and that it did not know that benefits had been granted to Claimant until January, 1978 when it received a Notice of Financial Determination Charged from the Bureau’s Harrisburg office. Upon receipt of the latter notice, Employer’s President contacted the local Bureau office and informed the personnel there that he had never received a copy of the initial determination. A copy of the November 28 notice was sent to him and, on January 23, 1978, Employer protested in writing the. Bureau’s decision.
Following the receipt of Employer’s protest, the Bureau revised its initial determination and denied unemployment compensation benefits to Claimant on the basis that he voluntarily terminated his employment with Employer without cause of a necessitous and compelling nature. See Section 402(b)(1) of the Unemployment Compensation Law (Law), Act of De*360cember 5, 1936, Second Ex. Sess., P.L. [1937] 2897, as amended, 43 P.S. §802(b)(l). In a second determination on tbe same date, tbe Burean held Claimant liable for a non-fault overpayment in the amount of $1,322.00. See Section 804(b) of the Law, 43 P.S. §874 (b). Claimant filed a timely appeal from those decisions to the Board. The Board concluded that the Bureau had no jurisdiction to reconsider its initial determination and, accordingly, reversed the referee’s decision on both the Section 402(b) (1) and the Section 804(b) issues. Employer then appealed to this Court. Upon stipulation by counsel for Employer and the Board, the proceedings were remanded for further testimony. Following a hearing,1 the Board issued the order from which Employer appeals here.
Section 501(e) of the Law, 43 P.S. §821(e), provides that
Unless the claimant or last employer or base-year employer of the claimant files an appeal with the board, from the determination contained in any notice required to be furnished by the department under section five hundred and one (a), (c) and (d), within fifteen calendar days after such notice was delivered to him personally, or was mailed to his last known post office address, and applies for a hearing, such determination of the department, with respect to the particular facts set forth in such notice, shall be final and compensation shall be paid or denied in accordance therewith. (Emphasis added.)
Section 509 of the Law, 43 P.S. §829, provides that
Any decision made by the department or any referee or the board shall not be subject to col*361lateral attack as to any application claim or claims covered thereby or otherwise be disturbed, unless appealed from.
Subject to appeal proceedings and judicial review, any right, fact or matter in issue which was directly passed upon or necessarily involved in any decision of a referee or the board or the Court and which has become final shall be conclusive for all purposes of this act and shall not be subject to collateral attack as among all affected parties who had notice of such decision: ....
The appeal provisions of the Law are mandatory. Failure to file an appeal within the required time, here fifteen days from the date of the Bureau’s determination, without an adequate excuse for the late filing, mandates that any appeal be dismissed and any collateral attack be denied. In order for a late appeal to be effective, the party appealing bears the burden of proving that he or she was deprived of the right to appeal by fraud or its equivalent, i.e. wrongful or negligent conduct by the administrative authorities. Strawley v. Unemployment Compensation Board of Review, 25 Pa. Commonwealth Ct. 34, 36, 358 A.2d 145, 147 (1976). See also Kitchell v. Unemployment Compensation Board of Review, 9 Pa. Commonwealth Ct. 149, 150-51, 305 A.2d 728, 729 (1973). Because the Board here entered an order adverse to Employer, the party who bore the burden of proof, our scope of review is limited to a determination of whether or not the Board’s findings of fact are consistent with each other and with its conclusions of law and its order and whether they can be sustained without a capricious disregard of competent evidence. Unemployment Compensation Board of Review v. DeVictoria, 24 Pa. Commonwealth Ct. 143, 146, 353 A.2d 920, 922 (1976).
*362The crucial question in this case is whether Employer was precluded from filing a timely appeal because it received no notice of the Bureau’s November 28 determination due to the wrongful or negligent conduct of the Bureau personnel.
The Board found as a fact that a copy of the November 28, 1977 determination was mailed to Employer at its last known post office address. Based upon this finding, the Board invoked the presumption of regularity of administrative acts of public officials in reaching the conclusion that Employer did have proper notice of the determination. The Board would prevail here if there were proof on the record that such notice was mailed to Employer at its last known post office address and that the notice was not returned to the Bureau. See Johnson v. Unemployment Compensation Board of Review, 42 Pa. Commonwealth Ct. 412, 414, 401 A.2d 4, 4 (1979). There is no such evidence. The Board’s finding concerning notice is based on a capricious disregard of competent evidence.
There is clear evidence on the record that the notice of determination of November 28 was mailed to Effort Foundry, Effort, Pa. 18353. The correct zip code was 18330. There is also evidence on the record that at some point in time the zip code on the Bureau’s record of such notice was corrected to read “18330.” Despite the Board’s assertion to the contrary, there is uncontradicted testimony by a Bureau official to indicate that the correction was made after the November 28 notice was sent. It is clear, then, that notice was not mailed to Employer’s last known post office address.2 Without that fact, the presump*363tion of regularity of administrative acts by public officials cannot stand, see, Waters v. New Amsterdam Casualty Co., 393 Pa. 247, 250-52, 144 A.2d 354, 356 (1958) and Leight v. Unemployment Compensation Board of Review, 49 Pa. Commonwealth Ct. 312, 314, 315, 410 A.2d 1307, 1309 (1980), and there is no evidence on the record to support a finding of notice to Employer.
We note the Board’s conclusion that the assertion made by the Employer’s representative that Employer did not receive notice of the November 28 determination was self-serving and lacking in credibility. Credibility of the witnesses and the weight to be given the evidence presented are, of course, matters to be decided by the compensation authorities. Unemployment Compensation Board of Review v. DeVictoria, 24 Pa. Commonwealth Ct. at 147, 353 A.2d at 922. Even discounting all of Employer’s testimony, however, it is clear from the record that Bureau personnel did not send notice of the November 28 declaration to Employer’s last known post office address. On the facts of this case, then, Sections 501(e) and 509 of the Law do not preclude Employer from appealing the November 28 determination.
The issue raised by Employer’s written protest filed in January, 1978 was whether Claimant was ineligible for unemployment compensation benefits because he voluntarily terminated his employment without cause of a necessitous and compelling nature. The Board has yet to consider this issue on its merits. Accordingly, we reverse the order of the Board and remand these proceedings to allow the Board to consider the eligibility issue.
*364Order
And Now, this 26th day of June, 1980, the Order of the Unemployment Compensation Board of Review, Decision No. B-156360-B, entered May 18, 1979, is reversed and the above-captioned matter is remanded for proceeding’s not inconsistent with this opinion.
Employer also asserts that it never received notice of the remand order entered by this Court or of the remand hearing before the Board. Employer’s counsel did receive notice of the hearing.
We need not decide whether the Butztown or Effort address would he considered Employer’s last known post office address. The notice was not sent to the Butztown address and it was not sent to the correct Effort address. The facts of this case are clearly distinguishable from those of Graul Unemployment Compensation Case, *363159 Pa. Superior Ct. 549, 49 A.2d 281 (1946), where the Superior Court held that the mailing of notice to S. S. Kresge Company’s retail store in Reading rather than to its main office in Detroit, Michigan was acceptable.