SYMPTOMS

When sending mail outs or sending emails via list server a remote server often Hotmail is returning "Too many recipients" after a certain point and dropping the connection.   This error can usually be seen in the MailEnable SMTP activity and/or debug logs.

CAUSE

Hotmail and some other domains have a spam protection method that prevents one connection from sending too many messages to their servers at one time. 

When MailEnable sends a group email or an internal message with many recipients (this includes lists) it will collate all the individual addresses for a domain and spawn a new message with all the recipients to this domain.  This can cause problems when sending to a server or domain that has a limit of incoming recipients per connection.

RESOLUTION

This behaviour is configurable within MailEnable via the following registry key you can configure a limit to how many messages can be sent to a domain within each connection.

HKEY_LOCAL_MACHINE\Software\Wow6432Node\Mail Enable\Mail Enable\Connectors\SMTP

"Maximum Recipients Per Domain"

The default amount is 256 you can lower this to be anything you like.

MORE INFORMATION

Note:  The lower the setting the higher the load hit for the SMTP service when it is splitting these messages up for sending.  Also the lower the limit configured the more messages that are being created.  Example a message with 300 recipients with this setting set to five will change a single message to 60 messages that all need to be sent. 

REFERENCES

MailEnable manual for configuration settings: https://www.mailenable.com/references.asp