SMTP Relay Settings: Host, Ports, TLS, Limits

AhaSend SMTP relay settings: send.ahasend.com and send-us.ahasend.com, ports 25/587/2525 with STARTTLS, PLAIN auth, message limits and SMTP error codes.

AhaSend SMTP relay settings are listed here: server hosts, STARTTLS ports, authentication, message limits and response codes.

Available on Free, Pro and Max. See plans and features.

New to SMTP? Check out our SMTP credentials guide to get started, or try our HTTP API for a more modern approach.

Which SMTP Settings Should I Use?

Use send.ahasend.com for the EU connection or send-us.ahasend.com for the US connection point. Both accept the same credentials and ports.

Setting	Value
EU host	send.ahasend.com
US connection point	send-us.ahasend.com; accepts SMTP and forwards it to the EU
Ports	25, 587 and 2525
Encryption	STARTTLS required on every supported port
Authentication	AUTH PLAIN only after STARTTLS
Username and password	Your SMTP credential, not an API key
Port 465	Implicit TLS is not supported

All message data is stored in the EU. See SMTP limits.

Does AhaSend Support Port 465?

No. AhaSend supports STARTTLS on ports 25, 587 and 2525. It does not support implicit TLS on port 465. Create an SMTP credential, then connect, start TLS and authenticate with AUTH PLAIN.

What Are the SMTP Limits?

Limit	Value	What to do
Recipients per message	50	Split larger sends into separate messages, or use the API.
Message size	20 MiB (20,971,520 bytes) for the complete message: headers, body and attachments after base64 encoding	Allow for base64 size growth when adding attachments. There is no separate per-attachment limit.
Messages per connection	10,000	Close and reopen the connection after reaching the limit; queued mail is unaffected.
Idle timeout	1 minute without new data	Reconnect after an idle or interrupted connection.

The recipient limit applies to consecutive RCPT TO commands in one SMTP transaction. The connection limit applies to consecutive MAIL FROM commands; extra commands receive a transient failure. The timeout also applies to slow transmission during DATA.

Base64 increases attachment size by about one third, so a 15 MB file uses about 20 MB before headers and message content are added. Messages over the limit are rejected during the SMTP transaction with a permanent size error (see SMTP errors); nothing is truncated. The same 20 MiB limit applies to inbound mail received for message routing. The API accepts request bodies up to 30 MB so that a maximum-size message still fits once JSON and base64 overhead are added.

Hourly sending limits start low for new accounts and grow with account age, plan and sending reputation; support can raise them. See facts and limits.

What Do SMTP Response Codes Mean?

AhaSend returns standard SMTP response codes with specific meanings:

Success Codes

Code	Description
250	OK - Message queued successfully for delivery

Error Codes

Code	Description	Action Required
421	Service Unavailable - This error could be returned as a result of disconnecting idle connections (idle too long), Load shedding (server is overwhelmed and cannot accept new connections), or temporary internal errors	Retry with exponential backoff
451	Internal server error - Temporary server issue	Retry the request
535 5.7.8	Invalid credentials - Authentication failed	Check username/password
521	Account disabled - Account suspended or out of credits	Check account status in dashboard
552	Invalid message format - Malformed email content	Validate email structure and headers
553	Email rejected - Recipient domain is toxic, temporary, or has typos	Verify recipient domain
556	Invalid account or domain - Account or sending domain not configured	Verify domain setup

553 email Rejection: This response is only returned if you enable Recipient Shields in your Account Settings.

How Do I Set Per-message Options?

AhaSend supports special email headers that modify system behavior:

Custom Headers

Learn about custom headers for tracking, routing, and advanced email features

Example Configurations

Basic SMTP Configuration

Host: send.ahasend.com
Port: 587
Security: STARTTLS
Username: your-smtp-username
Password: your-smtp-password
Authentication: PLAIN

Programming Language Examples

Node.js

Nodemailer and native SMTP examples

Python

smtplib and popular framework integrations

PHP

PHPMailer and framework examples

Go

Native Go SMTP implementations

.NET

C# and .NET Core examples

Command Line

sendmail, swaks, and curl examples

Best Practices

Connection Management

Optimize your SMTP connections:

• Reuse connections when sending multiple emails
• Implement connection pooling for high-volume sending
• Handle timeouts and reconnections gracefully
• Close connections properly when done

Error Handling

Handle SMTP errors appropriately:

• Implement exponential backoff for 421 responses
• Retry 451 errors with reasonable delays
• Don’t retry 5xx errors (permanent failures)
• Log error codes for debugging and monitoring

Performance Optimization

Maximize sending performance:

• Use persistent connections for batch sending (up to 10k messages per connection)
• Choose the server closest to your application
• Monitor connection limits and adjust accordingly

Security Considerations

Always use STARTTLS: Unencrypted SMTP connections are not secure and may expose your credentials and email content.

Credential Security

• Store SMTP credentials securely
• Use environment variables, not hardcoded values
• Rotate credentials regularly
• Use scoped credentials when possible

Domain Authentication

• Verify your sending domains
• Configure SPF, DKIM, and DMARC records
• Use dedicated sending domains
• Monitor domain reputation

Prefer HTTP API? While SMTP is great for compatibility, our HTTP API offers better performance, features, and error handling for modern applications.

Related Guides

• Domain setup, SMTP credentials, Special headers.
• Sandbox mode, Retention, API sending.
• SMTP CLI, Node.js SDK, Go SDK.
• Plans and limits, Tracking setup.