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When sending a message via a different SMTP outbound gateway, will a message containing multiple recipients in the BCC field be broken up into many messages by the originating server, or the server that is responsible for eventually sending the messages? For example, if I use Google but relay a 10-recipient message to another server over SMTP, does Google create 10 messages and send them via SMTP, or does the other server turn the original message into the 10 separate messages?

BACKGROUND: Using Google Apps for Work. We sometimes want to send emails to a large number of people at once, but would hit Google's sending limits.

We are considering using something like MailGun, which we can connect to via SMTP, to handle sending out mailing lists of up to 1000 recipients. Google Apps allows us to setup outbound gateways, but they also have limits.

I'm hoping that if we have an email with 1000 recipients in the BCC list, and relay it to MailGun, that Google will consider it as 1 email, and MailGun will have to do the "conversion" into 1000 actual email messages.

iopener
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  • Why the downvote?!? L'il help? – iopener Jun 14 '16 at 22:18
  • Mouse over the down arrow; the popup says "*This question does not show any research effort; it is unclear or not useful*". Downvotes without comment may be presumed to be for at least one of those reasons. – MadHatter Jun 15 '16 at 05:28

1 Answers1

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The RFCs generally recommend a limit of 100 recipient addresses per message. The mail server may expand this list if a recipient is a list known to the server. This applies to the envelope recipients, not the recipients listed on the header.

For a mailing lists, it is bad practice to list the recipients unless the message is being sent to a single user, and that is the only recipient listed.

Mail servers may list multiple recipients in the same domain when sending a message. Some or all of these may be rejected by the receiving server.

Setting up a list on your mail server will allow you to sent to and address the list address, and have that expanded by the client. Your client lists should not be accessible from the internet.

BillThor
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  • I appreciate this input, and apologize for my ignorance in this area. I don't know what envelope vs. header recipients are. And I'm not really sure how having lists "expanded by the client" would help here. You've given me some starting insight into email, but I'm not sure how this addresses the question. Is my question just a really bad one? – iopener Jun 14 '16 at 23:48
  • Like a paper letter, there are two parts the message and the envelope. The message has headers like Subject. From:,To:, Sender:, CC:, etc. Delivery does not depend on these headers, although the client will use the contents of the addressing header when submitting. These will be copied to the envelope which is used for delivery. – BillThor Jun 14 '16 at 23:52
  • OK, that's a good explanation--by client do you mean the mail app on my phone or do you mean the sending server? And further to addressing, does this mean that if the message header has multiple addresses, that multiple envelopes are created? Like, before the message is sent over SMTP relay? – iopener Jun 14 '16 at 23:59
  • By client I mean the mail app on your phone, or any other program you use to compose and send email. As the message is delivered the envelope changes. Your mail server will have all delivery addresses on its envelope. When it connects to another server, it will only add address for which that server is responsible. – BillThor Jun 15 '16 at 00:07
  • OK, that makes sense. But if my mail server is passing the message on via SMTP relay, does it just hand over an envelope containing all of the header addresses, or does split the message up into a bunch of envelopes for each recipient and then relay these over SMTP? Btw I'm sorry if this is an idiotic question I just really don't understand when in the process each step happens. – iopener Jun 15 '16 at 04:38
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    Find and read the RFCs. They describe the process. Also check http://www.systemajik.com/blog/running-an-email-server/ – BillThor Jun 15 '16 at 12:33