SMTP2GO API + MCP vs Zoho CPaaS (formerly ZeptoMail)

SMTP2GO API + MCP

API keys limited to chosen endpoints, with their own send rate. Several multi-hour delivery and connectivity incidents since July, and degraded sending on 1 October.

Zoho CPaaS (formerly ZeptoMail)

A public OpenAPI 3.0 file covers all 42 operations, and each Agent has its own send-only key. Email costs $2.50 per 10,000 with the first credit free. No API rate limit, 429 handling, idempotency key or SLA was found in the reviewed documentation, and new accounts wait for a manual review.

SMTP2GO API + MCP

1. Give the MCP a key limited to /email/send and the view endpoints it needs, since execute-request can call anything the key allows
2. Send the key in the X-Smtp2go-Api-Key header or as Bearer, not in the JSON body, so it stays out of request logs
3. Verify a sender domain first. Free accounts without one are held to 25 emails an hour
4. On 429, pause until calls return 200 again. Repeated errors time out your IP for at least a minute
5. Check smtp2gostatus.com before blaming your payload for mail stuck as 'processed'

Zoho CPaaS (formerly ZeptoMail)

1. Call the base URL for the account's region (cpaas.zoho.com, .eu, .in, .com.au, .jp or .com.cn). A key from one region fails on another
2. Send with Authorization: Zoho-enczapikey <key>. Logs, domains, templates, agents and suppressions need a Zoho OAuth token as Authorization: Zoho-oauthtoken <token>
3. Post OAuth client secrets and refresh tokens in the request body, not the query string the guide shows, so they stay out of logs
4. Don't retry a timed-out send blindly. There is no idempotency key, so look the message up by client_reference in the email logs first
5. Count one email per recipient, including cc and bcc, and keep each address field to 500 entries and each attachment to 15 MB
6. Use a sandbox Agent for tests. It accepts up to 10,000 emails a day and sends none of them