diff --git a/source/backend/api/Repositories/Ches/ChesRepository.cs b/source/backend/api/Repositories/Ches/ChesRepository.cs
index d350979e69..441618da1a 100644
--- a/source/backend/api/Repositories/Ches/ChesRepository.cs
+++ b/source/backend/api/Repositories/Ches/ChesRepository.cs
@@ -2,7 +2,6 @@
 using System.Net.Http;
 using System.Net.Http.Headers;
 using System.Net.Mime;
-using System.Security.Authentication;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Threading.Tasks;
@@ -12,6 +11,7 @@
 using Pims.Api.Models.CodeTypes;
 using Pims.Api.Models.Config;
 using Pims.Api.Models.Requests.Http;
+using Pims.Core.Api.Exceptions;
 using Polly.Registry;
 
 namespace Pims.Api.Repositories.Ches
@@ -98,33 +98,49 @@ public async Task<ExternalResponse<EmailResponse>> SendEmailAsync(EmailRequest r
                     _logger.LogError("CHES email send failed: {Status} {Reason} {Body}", response.StatusCode, response.ReasonPhrase, errorBody);
                     result.Message = $"CHES email send failed: {response.StatusCode} {response.ReasonPhrase}. Response body: {errorBody}";
                 }
+
+                return result;
             }
             catch (HttpRequestException ex)
             {
                 result.Status = ExternalResponseStatus.Error;
                 result.Message = $"HTTP error sending CHES email: {ex.Message}";
                 _logger.LogError(ex, "HTTP error sending CHES email.");
+
+                return result;
             }
             catch (TaskCanceledException ex)
             {
                 result.Status = ExternalResponseStatus.Error;
                 result.Message = $"Timeout sending CHES email: {ex.Message}";
                 _logger.LogError(ex, "Timeout sending CHES email.");
+
+                return result;
             }
             catch (JsonException ex)
             {
                 result.Status = ExternalResponseStatus.Error;
                 result.Message = $"Serialization error: {ex.Message}";
                 _logger.LogError(ex, "Serialization error sending CHES email.");
+
+                return result;
             }
             catch (AuthenticationException ex)
             {
-                result.Status = ExternalResponseStatus.Error;
+                result.Status = ExternalResponseStatus.NotExecuted;
                 result.Message = $"Authentication error: {ex.Message}";
                 _logger.LogError(ex, "Authentication error sending CHES email.");
+
+                return result;
             }
+            catch (Exception ex)
+            {
+                result.Status = ExternalResponseStatus.Error;
+                result.Message = $"Unexpected error sending CHES email: {ex.Message}";
+                _logger.LogError(ex, "Unexpected error sending CHES email.");
 
-            return result;
+                return result;
+            }
         }
 
         public async Task<HttpResponseMessage> TryGetHealthAsync()
diff --git a/source/backend/api/Services/NotificationUserService.cs b/source/backend/api/Services/NotificationUserService.cs
index ac58982115..ca55e4ebd3 100644
--- a/source/backend/api/Services/NotificationUserService.cs
+++ b/source/backend/api/Services/NotificationUserService.cs
@@ -57,8 +57,6 @@ public async Task PushNotificationUser(long notificationUserId)
             }
 
             userNotification.NotificationRetryCnt = userNotification.NotificationRetryCnt.HasValue ? ++userNotification.NotificationRetryCnt : 1;
-            var updatedNotification = _notificationUserOutputRepository.Update(userNotification);
-            _notificationUserOutputRepository.CommitTransaction();
 
             if (userNotification.NotificationOutputTypeCode == NotificationOutputTypes.EMAIL.ToString())
             {
@@ -77,24 +75,24 @@ public async Task PushNotificationUser(long notificationUserId)
                 {
                     case ExternalResponseStatus.Success:
                         {
-                            updatedNotification.NotificationSentDt = DateTime.UtcNow;
+                            userNotification.NotificationSentDt = DateTime.UtcNow;
                         }
                         break;
                     case ExternalResponseStatus.Error:
                     case ExternalResponseStatus.NotExecuted:
                         {
-                            updatedNotification.NotificationErrorDt = DateOnly.FromDateTime(DateTime.UtcNow);
-                            updatedNotification.NotificationErrorReason = chesResponse.Message;
+                            userNotification.NotificationErrorDt = DateOnly.FromDateTime(DateTime.UtcNow);
+                            userNotification.NotificationErrorReason = chesResponse.Message;
                         }
                         break;
                 }
             }
             else
             {
-                updatedNotification.NotificationSentDt = DateTime.UtcNow;
+                userNotification.NotificationSentDt = DateTime.UtcNow;
             }
 
-            _notificationUserOutputRepository.Update(updatedNotification);
+            _ = await _notificationUserOutputRepository.Update(userNotification);
             _notificationUserOutputRepository.CommitTransaction();
 
             return;
diff --git a/source/backend/dal/Repositories/Interfaces/INotificationUserOutputRepository.cs b/source/backend/dal/Repositories/Interfaces/INotificationUserOutputRepository.cs
index 024838cead..a601db0d14 100644
--- a/source/backend/dal/Repositories/Interfaces/INotificationUserOutputRepository.cs
+++ b/source/backend/dal/Repositories/Interfaces/INotificationUserOutputRepository.cs
@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Threading.Tasks;
 using Pims.Api.Models.Models.Concepts.Notification;
 using Pims.Dal.Entities;
 
@@ -10,6 +11,6 @@ public interface INotificationUserOutputRepository : IRepository
 
         public PimsNotificationUserOutput GetById(long notificationUserOutputId);
 
-        public PimsNotificationUserOutput Update(PimsNotificationUserOutput userNotification);
+        public Task<PimsNotificationUserOutput> Update(PimsNotificationUserOutput userNotification);
     }
 }
diff --git a/source/backend/dal/Repositories/NotificationUserOutputRepository.cs b/source/backend/dal/Repositories/NotificationUserOutputRepository.cs
index f67de4c77d..f59e132c2b 100644
--- a/source/backend/dal/Repositories/NotificationUserOutputRepository.cs
+++ b/source/backend/dal/Repositories/NotificationUserOutputRepository.cs
@@ -1,8 +1,9 @@
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Security.Claims;
+using System.Threading.Tasks;
 using LinqKit;
+using Medallion.Threading;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 using Pims.Api.Models.Models.Concepts.Notification;
@@ -14,12 +15,16 @@ namespace Pims.Dal.Repositories
 {
     public class NotificationUserOutputRepository : BaseRepository<PimsNotificationUserOutput>, INotificationUserOutputRepository
     {
+        private readonly IDistributedLockProvider _synchronizationProvider;
+
         public NotificationUserOutputRepository(
             PimsContext dbContext,
             ClaimsPrincipal user,
-            ILogger<DocumentRepository> logger)
+            IDistributedLockProvider synchronizationProvider,
+            ILogger<NotificationUserOutputRepository> logger)
             : base(dbContext, user, logger)
         {
+            _synchronizationProvider = synchronizationProvider;
         }
 
         public PimsNotificationUserOutput GetById(long notificationUserOutputId)
@@ -75,25 +80,21 @@ public PimsNotificationUserOutput GetById(long notificationUserOutputId)
                 .FirstOrDefault(x => x.NotificationUserOutputId == notificationUserOutputId) ?? throw new KeyNotFoundException();
         }
 
-        public PimsNotificationUserOutput Update(PimsNotificationUserOutput userNotification)
+        public async Task<PimsNotificationUserOutput> Update(PimsNotificationUserOutput userNotification)
         {
-            try
+            var @lock = _synchronizationProvider.CreateLock("NotificationUserOutputLock");
+            await using (await @lock.AcquireAsync())
             {
-                using var scope = Logger.QueryScope();
                 userNotification.ThrowIfNull(nameof(userNotification));
 
                 var existingUserNotification = Context.PimsNotificationUserOutputs
                     .FirstOrDefault(x => x.NotificationUserOutputId == userNotification.NotificationUserOutputId);
 
                 Context.Entry(existingUserNotification).CurrentValues.SetValues(userNotification);
+                Context.Update(existingUserNotification);
 
                 return existingUserNotification;
             }
-            catch(Exception ex)
-            {
-                Logger.LogError(ex.Message);
-                return null;
-            }
         }
 
         public IEnumerable<PimsNotificationUserOutput> GetAllByFilter(NotificationUserSearchFilterModel filter)
@@ -122,7 +123,7 @@ public IEnumerable<PimsNotificationUserOutput> GetAllByFilter(NotificationUserSe
 
             if(filter.NotificationTriggerDate is not null)
             {
-                predicateBuilder.And(x => x.NotificationUser.Notification.NotificationTriggerDate == filter.NotificationTriggerDate);
+                predicateBuilder.And(x => x.NotificationUser.Notification.NotificationTriggerDate <= filter.NotificationTriggerDate);
             }
 
             var query = Context.PimsNotificationUserOutputs