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Status Needs Clarification
Workspace Domino
Categories Administration
Created by Guest
Created on Jan 10, 2023

Router should be able to limit the number of messages per SMTP session when sending mails to a relay server

Currently when any antispam is used as a relay server, it may have the limit on how many messages/recipients it can accept in one SMTP session. Domino tries to send all messages/recipients in one SMTP session by default and thus it may be rejected with any error "SMTP Error 421: Too many messages in one session". So a mail with 105 recipients and antispam having a limit of 15 recipients per SMTP session may be rejected 7 times before it can be sent to final recipient. This causes a delay in transferring the message to a relay server. To avoid this delay, there is a feature request to provide the limit on how many messages/recipients router should send in one SMTP outbound session. This will allow to match the limit set by antispam for accepting messages.

The only workaround is to reduce the retry interval from Domino side that will set Domino to retry faster but the mails will be still rejected and there will be still delay in transferring mails.

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  • Guest
    Jan 16, 2023

    Here is the use case for this requirement -

    One client has purchased Barracuda Cloud Antispam for relaying their mails. This Barracuda Cloud Antispam as a fixed limit of accepting maximum 15 messages or 15 recipients in one SMTP session. This limit can't be increased by them and they can't add any exception to it. So now when Domino server is sending mail to Barracuda and if the mail contains 105 recipients then Barracuda rejects the remaining recipients after 15 recipients in each attempt. When the mail is rejected by Barracuda, then Domino retries that mail after 15 minutes and then sends mail to next 15 recipients. Next retry happens at 30 minutes and next 15 recipients get the mail. Similarly it takes 7 attempts to send this mail fully. What Barracuda expects is, we should send only 15 messages per SMTP session and then quit that session and then start a new session. Since the mail is rejected with an error "SMTP Error 421: Too many messages in one session" so Domino is retrying at scheduled retry interval and Domino is currently working as designed. Even if we reduce the retry interval but there would be still delay of several minutes in sending mail to last recipient as the mail will be still rejected by Barracuda. So this feature request is for providing a setting a limit the number of recipients to XX value per SMTP outbound session. This setting can be provided just below the setting "Relay host for messages leaving the local internet domain" so that clients can decide how many recipients they want to limit per SMTP session.

  • Admin
    Thomas Hampel
    Jan 13, 2023

    In case an SMTP session is interrupted, Domino will retry and so the delay should be a few seconds (if any).

    What is the expected business benefit of limiting the # of mails that can be sent in a single SMTP session? especially for outbound traffic that does'nt sound right.

    Neither this will reduce network traffic, nor it will provide any other obvious benefit. Actually it will even increase the network traffic because connecting clients (Domino connecting to your SMTP relay in this case) would have to re-authenticate every x mails which causes more network traffic.

    Can you please explain what the business benefit of this idea would be?