diff --git a/docker-compose.yml b/docker-compose.yml
index d29e7b1..a299122 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -107,6 +107,7 @@ services:
       MIMECAST_CLIENT_ID: ${MIMECAST_CLIENT_ID}
       MIMECAST_CLIENT_SECRET: ${MIMECAST_CLIENT_SECRET}
       MIMECAST_BASE_URL: ${MIMECAST_BASE_URL:-https://api.services.mimecast.com}
+      MIMECAST_ACCOUNT_CODE: ${MIMECAST_ACCOUNT_CODE}
 
       # IT Glue Configuration
       ITGLUE_API_KEY: ${ITGLUE_API_KEY}
diff --git a/lib/services/mimecast-client.ts b/lib/services/mimecast-client.ts
index 06b8e31..c55c1bb 100644
--- a/lib/services/mimecast-client.ts
+++ b/lib/services/mimecast-client.ts
@@ -15,6 +15,7 @@ export interface MimecastConfig {
   clientId: string;
   clientSecret: string;
   baseUrl?: string;
+  accountCode?: string;
 }
 
 interface TokenResponse {
@@ -82,12 +83,24 @@ export class MimecastClient {
   private readonly clientId: string;
   private readonly clientSecret: string;
   private readonly baseUrl: string;
+  private readonly accountCode: string;
   private tokenCache: TokenCache | null = null;
 
   constructor(config: MimecastConfig) {
     this.clientId = config.clientId;
     this.clientSecret = config.clientSecret;
     this.baseUrl = (config.baseUrl ?? 'https://api.services.mimecast.com').replace(/\/$/, '');
+    this.accountCode = config.accountCode ?? '';
+  }
+
+  /**
+   * Format a Date to the Mimecast-required format: yyyy-MM-ddTHH:mm:ss+0000
+   * Note: NO milliseconds — the API rejects .SSS variants
+   */
+  static formatDate(d: Date): string {
+    const pad = (n: number) => String(n).padStart(2, '0');
+    return `${d.getUTCFullYear()}-${pad(d.getUTCMonth() + 1)}-${pad(d.getUTCDate())}` +
+      `T${pad(d.getUTCHours())}:${pad(d.getUTCMinutes())}:${pad(d.getUTCSeconds())}+0000`;
   }
 
   // ── Auth ─────────────────────────────────────────────────────────────────
@@ -135,13 +148,18 @@ export class MimecastClient {
       url = `${url}?${new URLSearchParams(params).toString()}`;
     }
 
+    const headers: Record<string, string> = {
+      Authorization: `Bearer ${token}`,
+      'Content-Type': 'application/json',
+      Accept: 'application/json',
+    };
+    if (this.accountCode) {
+      headers['x-mc-account'] = this.accountCode;
+    }
+
     const res = await fetch(url, {
       method,
-      headers: {
-        Authorization: `Bearer ${token}`,
-        'Content-Type': 'application/json',
-        Accept: 'application/json',
-      },
+      headers,
       body: body ? JSON.stringify(body) : undefined,
     });
 
@@ -160,15 +178,19 @@ export class MimecastClient {
    * Response: { data: [{ trackedEmails: [...], pageToken: string }] }
    */
   async getMessageLogs(options: {
-    from: string;   // ISO datetime string
-    to: string;
+    from: Date;
+    to: Date;
     cursor?: string;
     pageSize?: number;
+    senderFilter?: string;
   }): Promise<PaginatedResult<MimecastMessage>> {
     const reqData: Record<string, any> = {
-      from: options.from,
-      to: options.to,
-      pageSize: options.pageSize ?? 500,
+      accountCode: this.accountCode,
+      advancedTrackAndTraceOptions: {
+        from: options.senderFilter ?? '',
+      },
+      start: MimecastClient.formatDate(options.from),
+      end: MimecastClient.formatDate(options.to),
     };
     if (options.cursor) reqData.pageToken = options.cursor;
 
@@ -380,7 +402,7 @@ export class MimecastClient {
    */
   async testConnection(): Promise<{ ok: boolean; accountName?: string; packageName?: string; error?: string }> {
     try {
-      const data = await this.request<any>('POST', '/api/account/get-account', { data: [{}] });
+      const data = await this.request<any>('POST', '/api/account/get-account', { data: [{ accountCode: this.accountCode }] });
       const account = data.data?.[0] ?? {};
       return {
         ok: true,
@@ -406,6 +428,7 @@ export function getMimecastClient(): MimecastClient {
       clientId,
       clientSecret,
       baseUrl: process.env.MIMECAST_BASE_URL ?? 'https://api.services.mimecast.com',
+      accountCode: process.env.MIMECAST_ACCOUNT_CODE ?? '',
     });
   }
   return _client;
diff --git a/lib/services/mimecast-sync-service.ts b/lib/services/mimecast-sync-service.ts
index aae47f5..977127b 100644
--- a/lib/services/mimecast-sync-service.ts
+++ b/lib/services/mimecast-sync-service.ts
@@ -196,12 +196,9 @@ export async function syncMimecastMessages(
   let total = 0;
   let cursor: string | null = null;
 
-  const from = fromDate.toISOString();
-  const to = toDate.toISOString();
-
   do {
     try {
-      const result = await client.getMessageLogs({ from, to, cursor: cursor ?? undefined, pageSize: 500 });
+      const result = await client.getMessageLogs({ from: fromDate, to: toDate, cursor: cursor ?? undefined, pageSize: 500 });
       if (result.items.length > 0) {
         total += await upsertMessages(result.items);
       }