--- title: Inbound SMTP Restrictions description: In order to provide the highest level of service our inbound processing infrastructure has some restrictions. Below are guidelines and limits you should follow to ensure the best results when submitti --- [Skip to content](https://help.socketlabs.com/docs/inbound-smtp-restrictions#main-content) English Show submenu for translations [Contact us](https://help.socketlabs.com/docs/kb-tickets/new?hsLang=en) [![SocketLabs logo](https://help.socketlabs.com/hs-fs/hubfs/Logos/SocketLabs%20logo.png?width=243&height=70&name=SocketLabs%20logo.png)](https://www.socketlabs.com/) Open main navigation Close main navigation - English Show submenu for translations - [Contact us](https://help.socketlabs.com/docs/kb-tickets/new) SocketLabs is here to help! - There are no suggestions because the search field is empty. 1. [Help Center](https://help.socketlabs.com/docs?hsLang=en) 2. [Advanced](https://help.socketlabs.com/docs/advanced?hsLang=en) # Inbound SMTP Restrictions ### Inbound SMTP Restrictions In order to provide the highest level of service our inbound processing infrastructure has some restrictions. Below are guidelines and limits you should follow to ensure the best results when submitting messages for delivery via SocketLabs. -SocketLabs recommends configuring applications to open up to 20 simultaneous SMTP connections to smtp.socketlabs.com Numerous factors impact the absolute maximum number of connections available to customers at a given time. Customers with implementations that require more the 20 concurrent connections should contact SocketLabs Support.  -Maximum message size, including any embedded images and attachments, is 40MB -Maximum 500 total recipients per message ### More Information See our other articles for more information on related topics. IP Addresses, Ports, and more inbound SMTP connection information: [SMTP Connections](https://help.socketlabs.com/docs/smtp-connections?hsLang=en) To better troubleshoot and diagnose SMTP connectivity issues see our blog on [Diagnosing SMTP Service Connection Issues](http://www.socketlabs.com/blog/diagnosing-smtp-service-connection-issues/)  ### Related Errors If your message is larger than 40MB you will receive the following error: [552 5.3.4 - Message too big for system](https://help.socketlabs.com/docs/552-534-message-too-big-for-system?hsLang=en) If you attempt to send to more than 500 addresses in one message you will receive the following error: [452 5.5.3 - Too many recipients](https://help.socketlabs.com/docs/452-553-too-many-recipients?hsLang=en) If you attempt to open an excessive number of connections you will receive the following error: [421 4.4.5 - Too many connections from your host](https://help.socketlabs.com/docs/421-445-too-many-connections-from-your-host?hsLang=en) - [Fundamentals](https://help.socketlabs.com/docs/fundamentals?hsLang=en#main-content) - [StreamScore](https://help.socketlabs.com/docs/fundamentals?hsLang=en#streamscore) - [Reports](https://help.socketlabs.com/docs/fundamentals?hsLang=en#reports) - [Frequently Asked Questions](https://help.socketlabs.com/docs/fundamentals?hsLang=en#frequently-asked-questions) - [Advanced](https://help.socketlabs.com/docs/advanced?hsLang=en#main-content) - [Security](https://help.socketlabs.com/docs/advanced?hsLang=en#security) - [DNS Authentication](https://help.socketlabs.com/docs/dns-authentication?hsLang=en#main-content) - [DKIM](https://help.socketlabs.com/docs/dns-authentication?hsLang=en#dkim) - [SPF and Custom Bounce Domain](https://help.socketlabs.com/docs/dns-authentication?hsLang=en#spf-and-custom-bounce-domain) - [Deliverability](https://help.socketlabs.com/docs/deliverability?hsLang=en) - [For Developers](https://help.socketlabs.com/docs/for-developers?hsLang=en#main-content) - [Event Webhooks](https://help.socketlabs.com/docs/for-developers?hsLang=en#event-webhooks) - [Integrations](https://help.socketlabs.com/docs/integrations?hsLang=en#main-content) - [Languages and Frameworks](https://help.socketlabs.com/docs/integrations?hsLang=en#languages-and-frameworks) - [Mail Servers](https://help.socketlabs.com/docs/integrations?hsLang=en#mail-servers) - [Web Clients and Plug-ins](https://help.socketlabs.com/docs/integrations?hsLang=en#web-clients-and-plug-ins) - [Mail Clients](https://help.socketlabs.com/docs/integrations?hsLang=en#mail-clients) - [Marketing Tools](https://help.socketlabs.com/docs/marketing-tools?hsLang=en#main-content) - [Tips and Tricks](https://help.socketlabs.com/docs/marketing-tools?hsLang=en#tips-and-tricks) - [Troubleshooting](https://help.socketlabs.com/docs/troubleshooting?hsLang=en#main-content) - [SocketLabs Inbound SMTP Gateway Errors](https://help.socketlabs.com/docs/troubleshooting?hsLang=en#socketlabs-inbound-smtp-gateway-errors) - [SocketLabs Outbound SMTP Errors](https://help.socketlabs.com/docs/troubleshooting?hsLang=en#socketlabs-outbound-smtp-errors) - [Spotlight Email Analytics](https://help.socketlabs.com/docs/spotlight-email-analytics?hsLang=en) - [Hurricane MTA](https://help.socketlabs.com/docs/hurricane-mta?hsLang=en#main-content) - [Advanced Hurricane Usage](https://help.socketlabs.com/docs/hurricane-mta?hsLang=en#advanced-hurricane-usage) - [Upgrading Hurricane](https://help.socketlabs.com/docs/hurricane-mta?hsLang=en#upgrading-hurricane) - [Troubleshooting](https://help.socketlabs.com/docs/hurricane-mta?hsLang=en#troubleshooting) - [Hurricane Maintenance](https://help.socketlabs.com/docs/hurricane-mta?hsLang=en#hurricane-maintenance) - [Getting Started with Hurricane](https://help.socketlabs.com/docs/hurricane-mta?hsLang=en#getting-started-with-hurricane) - [Basic Hurricane Usage](https://help.socketlabs.com/docs/hurricane-mta?hsLang=en#basic-hurricane-usage) - [Complex Sender](https://help.socketlabs.com/docs/complex-sender?hsLang=en) [![Chill listening crop-3](https://help.socketlabs.com/hs-fs/hubfs/Logos/Logo_Favicon%20-%20large%20-%20cropped.png?width=24&height=24&name=Logo_Favicon%20-%20large%20-%20cropped.png "Chill listening crop-3")](https://www.socketlabs.com) Copyright © 2026, SocketLabs LLC