Email Provider Quotas, Rate Limits & Protocol Specifications

This technical reference provides an exhaustive specification of daily sending volumes, rate-limiting behavior, network transport protocols, and authentication mechanics supported by FormMail Hub. It covers both default Google Workspace transport limits and external transactional SMTP providers.

1. Transport Engine Architecture & Quota Allocation

FormMail Hub employs an intelligent dual-routing dispatch architecture designed to optimize sending limits and eliminate unnecessary quota consumption.

+-----------------------------------------------------------------------------------+
|                             FORM MAIL HUB DISPATCH ENGINE                         |
+-----------------------------------------------------------------------------------+
                                  |                      |
          +-----------------------+                      +-----------------------+
          |                                                              |
          v                                                              v
+-----------------------------------+                          +-----------------------------------+
|      INTERNAL SYSTEM RELAY        |                          |     RESPONDENT AUTO-RESPONDERS    |
|   (SYSTEM ALERTS & REPORTS ONLY)  |                          |      & BROADCAST CAMPAIGN ENGINE   |
+-----------------------------------+                          +-----------------------------------+
| Routing: System Relay Infrastructure|                          | Routing: User Custom SMTP         |
| Scope: Submission Alerts, Daily   |                          | (Amazon SES, SendGrid, Mailgun,   |
|   Reports, SMTP Connection Errors |                          |  Google Workspace App Password)   |
| Target: Admins, Reviewers, Teams  |                          | Target: Respondents & Lead Lists  |
+-----------------------------------+                          +-----------------------------------+

Dual-Routing System Breakdown

Internal System Relay: FormMail Hub routes administrative system alerts (submission notifications, daily activity reports, and SMTP connection error alerts) through dedicated infrastructure. This guarantees 0% consumption of your personal or organization’s daily SMTP quota.
Custom SMTP Transport: Client-facing emails (auto-responders, multi-step triggers, and bulk marketing campaigns) run strictly through your configured SMTP server, granting full ownership over domain reputation, DKIM signing, and daily sending volume.

2. Provider Capability & Protocol Matrix

The following matrix outlines standard capabilities, connection ports, default limits, and feature support across all supported email dispatch channels:

Email Provider / Route	Standard Daily Quota	Max Burst Rate	Supported Ports	Security Protocol	Broadcast Campaigns Supported?
Amazon SES	10,000 to 1,000,000+	Up to 500 msgs/sec	587, 465, 2525	STARTTLS / TLS	✅ Yes (Sheets Core Only)
SendGrid (Twilio)	100/day (Free) to Unlimited	Provider Tiered	587, 465, 2525	STARTTLS / TLS	✅ Yes (Sheets Core Only)
Mailgun	Account Tiered	100 msgs/sec	587, 465, 2525	STARTTLS / TLS	✅ Yes (Sheets Core Only)
Postmark	Account Tiered	300 msgs/sec	587, 25, 2525	STARTTLS / TLS	✅ Yes (Sheets Core Only)
Google Workspace SMTP	1,500 / day / user	~1 msg/sec	587, 465	STARTTLS / TLS	✅ Yes (Sheets Core Only)
Free Gmail™ SMTP	500 / day / user	~1 msg/sec	587, 465	STARTTLS / TLS	⚠️ Limited (Max 500/day)
Custom Corporate SMTP	Server-Defined	Server-Defined	587, 465, 25	STARTTLS / SSL	✅ Yes (Sheets Core Only)

Note on Campaign Support: As detailed in the Ecosystem Architecture, bulk email broadcast campaigns require contact list indexing stored in Google Sheets cell vectors. Consequently, campaign dispatch is ONLY enabled when launching FormMail Hub from the Google Sheets core add-on.

3. Detailed Provider Technical Specifications

A. Amazon SES (Simple Email Service)

Authentication Mechanism: IAM User Credentials (Access Key ID & Secret Key generated under SMTP Settings).
Hostname Convention: email-smtp.{region}.amazonaws.com (e.g., email-smtp.us-east-1.amazonaws.com).
Sandbox Restrictions: New SES accounts are placed in the AWS Sandbox environment. Outbound emails are restricted to verified recipient addresses until a Production Access Request is approved in the AWS Console.
Rate Limiting: Default production quota begins at 14 emails/second and scales automatically up to several hundred messages per second based on bounce/complaint metrics.

B. SendGrid

Authentication Mechanism: API Key authentication using the fixed username apikey and the generated API Key string as the password.
Hostname: smtp.sendgrid.net
Port Configuration: Recommended Port 587 with TLS encryption.
Sender Verification: Requires Single Sender Verification or full Domain Authentication (CNAME DNS records for SPF/DKIM alignment).

C. Google Workspace & Gmail™ App Passwords

Authentication Mechanism: 16-character Google App Password (requires 2-Step Verification enabled on the Google Account).
Hostname: smtp.gmail.com
Port Configuration: Port 587 (TLS) or Port 465 (SSL).
Quota Escalation Rules:
- Trial Workspace Accounts: Restricted to 500 emails/day.
- Paid Workspace Accounts: Up to 1,500 emails/day.
- Recipient Limits: A single email sent to 50 CC/BCC recipients counts as 50 separate units against the daily quota.

D. Custom On-Premise & Corporate SMTP Relays

Supported Encryption: STARTTLS, explicit TLS, and SSL.
IP Whitelisting Requirements: If your corporate firewall restricts outbound relays, ensure standard Google Cloud Apps Script egress IP ranges are permitted.

4. Rate Limiting, Queueing & Batch Execution Engine

To prevent provider connection timeouts and adhere to remote rate limits, FormMail Hub executes outgoing email requests through an automated queuing engine:

[ Submit Event / Campaign Schedule ]
                 |
                 v
     [ Queue Buffering Stage ]
                 |
                 +---> Check Batch Chunk Size (e.g., 50 recipients)
                 |
                 v
    [ Execute Remote SMTP Handshake ]
                 |
        +--------+--------+
        |                 |
        v                 v
   (Success 250)    (Rate Limited 421/451)
        |                 |
        v                 v
[ Log Delivered ]   [ Exponential Backoff Delay ]
                          |
                          v
                   [ Re-queue Job ]

Queue Execution Mechanics

Batch Chunking: Outbound bulk broadcasts are segmented into discrete execution chunks to stay within Google Apps Script runtime constraints (6 minutes max execution duration per thread).
Exponential Backoff: If a remote SMTP server returns temporary soft error codes (421 Try again later, 451 Transient Local Error, or 452 Too many recipients), FormMail Hub automatically buffers the unsent messages and reschedules execution across exponential backoff intervals.
Hard Failure Circuit Breaker: If a connection encounters standard authentication failures (535 Authentication Credentials Invalid) or hard bounces (550 Recipient Unknown), dispatch to that target is halted immediately and logged in the Audit Log.

5. Network Security, Ports & TLS Encryption Standards

FormMail Hub supports secure socket connections across all industry-standard ports:

Port 587 (Recommended): Uses STARTTLS. The connection begins as unencrypted text and upgrades to secure TLS/SSL before credentials or payload data are transmitted.
Port 465: Uses Implicit SSL/TLS. The connection is encrypted from the initial TCP handshake.
Port 2525: Alternative TLS port commonly used when cloud hosting providers or local ISPs block standard outbound Port 25.
Port 25 (Legacy): Supported but not recommended due to widespread ISP egress filtering.

6. Diagnostic Quota Escalation & Remediation Matrix

Symptom / Error Code	Root Cause	Technical Remediation Strategy
`ERR_QUOTA_EXCEEDED`	Google Workspace daily email limit hit via native relay.	Switch delivery route to Custom SMTP (Amazon SES or SendGrid) in Settings > SMTP Config.
`452 4.5.3 Too many recipients`	Provider single-transaction recipient limit exceeded.	Enable batch mode in FormMail Hub settings to deliver messages as individual transactions.
`535 5.7.8 Auth Failed`	Google Account password used instead of App Password.	Enable 2FA on Google Account and generate a dedicated 16-character App Password.
`SES Sandbox Limit Reached`	AWS SES account restricted to sandbox recipient list.	Submit an AWS SES Production Access request in the AWS SES Console.

7. Official Resources & Contact

Google Workspace Marketplace: Install via the Google Workspace Marketplace Listing.
Live Demo Form: Test submission triggers on our Live Demo Form.
Technical Support: Request assistance at Support & Contact.
Legal Policies: Review our Privacy Policy and Terms of Service.

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