Quote:

...If I compose an email with a few attachments and send it to a friend with several Ccs to other fiends, what happens to it?
Do the attachments get up-loaded once to the ISP and then sent to each of the Ccs, or do they get up-loaded with each of the Ccs before sending?
I'm asking this question because I believe that one cause of the poor performance of our retirement village network is that members keep sending on emails that they receive as I have outlined above. On networks, once upload bandwidth hits maximum, download bandwidth drops to a crawl.



An outgoing email with multiple recipients is sent to the mail server as a single communication. I.E. The mail server gets one copy but sends to each CC or BC or TO recipient individually.

So, if the mail server is at your ISP, then you send a single copy to the mail server via your internet link using the SMTP protocol*, it in turn will use the SMTP protocol to send each recipient a copy to each recipients mail server. IIRC, even if there are several recipients on one post office, so long as it ISN'T the mail server you use for your outgoing email, then multiple copies will be sent.

This also applies if the mail server is local. I.E. hosted at your end of the internet service rather than at the ISP's end. However in this case, the mail server will send multiple copies up your link as you have moved the mail server function to your local end.

Poor link performance and slow outgoing emails is usually caused by exceeding the bandwidth capability of the link. Bear in mind that an ADSL service is Asymetric. I.E. the upload speed is a fraction of the download speed**. This can also affect downlink performance as requests for data can be seriously delayed by large quantities of outgoing data. As a result of this phenomena, we often set up our customers with routers that have QoS (Quality of Service) controls and use this to limit the outbound SMTP priority.

It is surprising how many people have absolutely no feel for the quantity of data they are actually sending...


*There are exceptions, you could be using IMAP or a proprietry protocol, but in general it will be SMTP.

** A 1500/256 link is 1.5 Mbit down and only 256kbit up - a 6 to 1 ratio. ADSL 2 and 2+ links are typically greater in the ratio - on the order of 8 to 1 and ranging up to 20 to 1. Assuming you're actually achieving full speed on your link!

--=-- My credo: If it's worth doing, it's worth overdoing - just ask my missus, she'll tell ya laugh --=--
You're only paranoid if you're wrong!