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I'm currently writing an email client for fun/practice and I've noticed that after sending out a handful of test emails (10 or so) over a short period of time, the emails stop going through. At some point later I'm able to send again from the client and the process starts over.
Is it possible that my ISP's mail server is detecting me and temporarily blocking me? And if that is the case, why are they doing that, and is there another way for me to test my mail client?
Does my ISP's mail server single me out as a possible spammer because I haven't registered this IP address as a mail server with them? – Ryan – 2011-05-13T07:18:34.607
1Probably not, because most ISPs just arbitrarily block outbound TCP port 25 traffic to everywhere except their own internal eMail servers (so their users can send eMail) -- this prevents what is called "direct-to-MX mailing" by spammers using Spamware designed to send eMail directly to each victim's SMTP server, and it also prevents open SMTP relay abuse (another technique used by spammers to abuse the internet). If you do need to run an SMTP server, then you'll need to get a Static IP address from your ISP and have a PTR record configured in their DNS ARPA zone (yeah, "registered"). – Randolf Richardson – 2011-05-13T07:21:59.023
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@Ryan Yes, there are a couple of other things to consider too. An MX record would help. What other ISPs may do is "ask" if your IP address really belongs to a registered mail server. Also, a RFC-conform client that sends its correct host name is sometimes needed. Or a client that correctly communicates with greylisting servers. As I said, there are lots of things nowadays :)
– slhck – 2011-05-13T07:24:50.973I wasn't looking to run my own mail server. I mainly just wanted to write my own email client and use it. Is there a way I can do just that without having the ~10 email limit before timeout? – Ryan – 2011-05-13T07:31:13.653
1@Ryan As @Randolf said, just test it locally, Mercury is good, if you can run Linux there are a couple of alternatives as well. – slhck – 2011-05-13T08:13:05.327
Right! Sorry, my mind was stuck on what I'm doing not being a mail server, when in reality it acts like one. A true mail client would only interact with a mail server, not generate its own emails. Thank you! – Ryan – 2011-05-13T22:15:07.217