SMTP Relay Settings: Host, Ports, TLS, Limits

AhaSend SMTP relay settings are listed here: server host, ports and encryption, 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 with your SMTP credentials on port 587, 25 or 2525 with STARTTLS, or on port 465 with implicit TLS.

Setting	Value
Host	`send.ahasend.com`
Ports with STARTTLS	25, 587 and 2525
Port with implicit TLS	465 (TLS from the first byte, also called SSL/TLS or SMTPS)
Encryption	Required on every port: STARTTLS on 25, 587 and 2525, implicit TLS on 465
Authentication	AUTH PLAIN after STARTTLS on 25, 587 and 2525; AUTH PLAIN or AUTH LOGIN on 465
Username and password	Your SMTP credential, not an API key

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

Does AhaSend Support Port 465?

Yes. Port 465 uses implicit TLS: your client starts TLS as soon as it connects, before the server greets it, and sends no STARTTLS command. It accepts AUTH PLAIN and AUTH LOGIN. Limits, response codes and special headers are the same on every port. Set your client’s security option to match the port:

Library or tool	Port 465	Ports 587, 25 and 2525
Nodemailer	`secure: true`	`secure: false`
Python `smtplib`	`smtplib.SMTP_SSL`	`smtplib.SMTP`, then `starttls()`
PHPMailer	`PHPMailer::ENCRYPTION_SMTPS`	`PHPMailer::ENCRYPTION_STARTTLS`
MailKit	`SecureSocketOptions.SslOnConnect`	`SecureSocketOptions.StartTls`
Swaks	`--tls-on-connect`	`--tls`

A client set to STARTTLS on port 465 waits for a greeting that never comes, and the server closes the connection after 30 seconds. A client set to SSL/TLS on port 587 fails at once with a TLS error such as wrong version number. See port 465 problems.

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 - Sending paused or out of credits	Check account status in dashboard. See paused accounts
`550 5.7.1`	Domain paused - Sending from the sender domain is paused	See paused domains
`552 5.6.0`	Invalid message format - The message is malformed, for example it uses bare line feeds instead of CRLF line endings or has no `From` header	Fix the message structure and headers named in the reply, then resend
`552 5.3.4`	Message too large - The message exceeds the 20 MiB size limit	Reduce content or attachments, then resend
`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

Port 465 Configuration

Host: send.ahasend.com
Port: 465
Security: SSL/TLS (implicit TLS)
Username: your-smtp-username
Password: your-smtp-password
Authentication: PLAIN or LOGIN

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 TLS: use STARTTLS on ports 25, 587 and 2525, or implicit TLS on port 465. 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.

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

Getting Started

Domain Authentication

Delivery and Troubleshooting

Sending with SMTP

Sending with the API

Campaign Builder

Email Tracking

Integrations

Data Retention

Security

Scaling up

SMTP Relay Settings: Host, Ports, TLS, Limits

Which SMTP Settings Should I Use?

Does AhaSend Support Port 465?

What Are the SMTP Limits?

What Do SMTP Response Codes Mean?

Success Codes

Error Codes

How Do I Set Per-message Options?

Custom Headers

Example Configurations

Basic SMTP Configuration

Port 465 Configuration

Programming Language Examples

Node.js

Python

PHP

Go

.NET

Command Line

Best Practices

Security Considerations

Credential Security

Domain Authentication

​Which SMTP Settings Should I Use?

​Does AhaSend Support Port 465?

​What Are the SMTP Limits?

​What Do SMTP Response Codes Mean?

​Success Codes

​Error Codes

​How Do I Set Per-message Options?

Custom Headers

​Example Configurations

​Basic SMTP Configuration

​Port 465 Configuration

​Programming Language Examples

Node.js

Python

PHP

Go

.NET

Command Line

​Best Practices

​Security Considerations

Credential Security

Domain Authentication

​Related Guides

Which SMTP Settings Should I Use?

Does AhaSend Support Port 465?

What Are the SMTP Limits?

What Do SMTP Response Codes Mean?

Success Codes

Error Codes

How Do I Set Per-message Options?

Example Configurations

Basic SMTP Configuration

Port 465 Configuration

Programming Language Examples

Best Practices

Security Considerations

Related Guides