diff --git a/dictation_server/src/common/test/overrides.ts b/dictation_server/src/common/test/overrides.ts
index 6be0936..a24758c 100644
--- a/dictation_server/src/common/test/overrides.ts
+++ b/dictation_server/src/common/test/overrides.ts
@@ -71,6 +71,16 @@ export const overrideSendgridService = <TService>(
       value: sendMail,
       writable: true,
     });
+  } else {
+    // [重要]
+    // sendMailだけは外部に対する送信を行ってしまう & 失敗によりメールアドレス自体の信頼度が変動してしまうため、
+    // overrideした場合には"必ず"偽物の呼び出しになるようにしておく
+    Object.defineProperty(obj, sendMail.name, {
+      value: async () => {
+        return;
+      },
+      writable: true,
+    });
   }
 
   if (createMailContentFromEmailConfirm) {
diff --git a/dictation_server/src/features/accounts/accounts.service.spec.ts b/dictation_server/src/features/accounts/accounts.service.spec.ts
index 2ffaf96..f2a5ad0 100644
--- a/dictation_server/src/features/accounts/accounts.service.spec.ts
+++ b/dictation_server/src/features/accounts/accounts.service.spec.ts
@@ -69,7 +69,14 @@ describe('createAccount', () => {
         return { sub: externalId };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirm: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     const { accountId, externalUserId, userId } = await service.createAccount(
       companyName,
@@ -110,7 +117,14 @@ describe('createAccount', () => {
       },
     });
 
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirm: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     const companyName = 'test_company_name';
     const country = 'US';
@@ -159,7 +173,14 @@ describe('createAccount', () => {
       },
     });
 
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirm: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     const companyName = 'test_company_name';
     const country = 'US';
diff --git a/dictation_server/src/features/users/users.service.spec.ts b/dictation_server/src/features/users/users.service.spec.ts
index 848b41f..b694b2b 100644
--- a/dictation_server/src/features/users/users.service.spec.ts
+++ b/dictation_server/src/features/users/users.service.spec.ts
@@ -403,7 +403,14 @@ describe('UsersService.createUser', () => {
         return { sub: externalId };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     expect(
       await service.createUser(
@@ -478,7 +485,14 @@ describe('UsersService.createUser', () => {
         return { sub: externalId };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     expect(
       await service.createUser(
@@ -556,7 +570,14 @@ describe('UsersService.createUser', () => {
         return { sub: externalId };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     expect(
       await service.createUser(
@@ -631,7 +652,14 @@ describe('UsersService.createUser', () => {
         return { sub: externalId };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     expect(
       await service.createUser(
@@ -701,7 +729,14 @@ describe('UsersService.createUser', () => {
         return { sub: externalId };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     // DBエラーを発生させる
     overrideUsersRepositoryService(service, {
@@ -765,7 +800,14 @@ describe('UsersService.createUser', () => {
         throw new Error('ADB2C error');
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     try {
       await service.createUser(
@@ -830,7 +872,14 @@ describe('UsersService.createUser', () => {
         };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     try {
       await service.createUser(
@@ -897,7 +946,14 @@ describe('UsersService.createUser', () => {
         return { sub: externalId_1 };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     expect(
       await service.createUser(
@@ -1009,7 +1065,14 @@ describe('UsersService.createUser', () => {
         return { sub: externalId };
       },
     });
-    overrideSendgridService(service, {});
+    overrideSendgridService(service, {
+      sendMail: async () => {
+        return;
+      },
+      createMailContentFromEmailConfirmForNormalUser: async () => {
+        return { html: '', text: '', subject: '' };
+      },
+    });
 
     // AuthorIDのUNIQUE制約エラーを発生させる
     overrideUsersRepositoryService(service, {