Issue with bulk email delievery

It looks like you're facing intermittent email delivery issues with the 451 4.7.500 Access denied error. Here are some quick fixes:

1. Check Sending Limits: Ensure you’re not exceeding your server’s sending limits.
2. IP Reputation: Your IP might be blacklisted; use tools like MXToolbox to check.
3. SMTP Configuration: Review settings for any misconfigurations or blocks.
4. Retry Mechanism: Implement automatic retries for undelivered emails.
5. Dedicated SMTP: Consider switching to a dedicated SMTP server to improve deliverability.

Some reliable SMTP services:

• SMTPmart
• SMTPget
• SendGrid
• iDealSMTP
• Amazon SES

These can help improve email delivery and avoid blocks.

Thanks for your valuable inputs guys!!!

@Charlie Arehart Yes, I am aware about mail sending limits on my mail server. As I mentioned, in my situation sometimes at same milisecond email will deliever fine, and next might stuck into undeliver emails. So, if there's any limit issues then all the emails should stuck on server one after another and none of next should get deliever. To add more, we have 5000 emails limit per hour set on our mail server(as I mentioned there are cases where 300 emails also cannot deliver successfully).
Also, SMTP Access denied error is also weird. Not sure why suddenly it cannot access SMTP server to send emails.
So, seems solutions mentioned by @sdsinc_pmascari and @rickmaz  are only possible workarounds here 😞

Thanks once again!

We have had that problem for the reasons you mentioned but also just random reasons like your mail server was blipping or being rebooted or under maintenance at the time you were sending mail. Whatever, they sit in the undeliverable folder, and if you don't manually check you have no idea. So what we did was the following:

1. Since most issues will be resolved in our application and experience by simply resending we have code that checks the folder and sees if anything is in there first like every 2 hours. If it does we will move it back to the spool folder (via cf code). If at the end of the day or whenver there is still mail in there then something keeps failing and this could be for many reasons so we want to inspect.

2. So at the end of the day we run another cf script that alerts us only if there is anything in folder, outputs the email content so we can see what the issue may be, and can either fix it and save (all through cf code) or we can retry the entire spool or delete the message if it was just something that was a bad email or not needing to be sent. 

So we basically never have to log in to the cf server and it has worked out fairly well. 

This is not a cf error: your mail server is throttling you because you're hitting some rate limits it imposes. (This is about the mail server which you have cf pointing to, either in the cfmail tag or the cf admin. And to be clear, that's not the USER'S mail server, in the to address. It's instead like your local post office, which takes mail you send and delivers it ultimately to the recipient. CF knows nothing about that process. )

The key indicator is the 451 code in that error. If you Google:

smtp 451

You will find this discussed extensively (though again having nothing to do with cf).

Naturally you'll ask "what can I do?"

1) Your first choice should be to look into that mail server: maybe there's some configurability for that rate limit. Or maybe it will change if you register your cf server's ip address to be trusted. Or maybe you can pay some amount to be granted the ability to send more mail at once.

2) You could change mail server providers. Different ones have different limits. You could also configure a local smtp server instead, which suits many. There are several alternatives. More on finding alternatives in a moment. 

And FWIW, with CF Enterprise you can name multiple mail servers for cf to use. 

3) Most people in your shoes seek to somehow limit or manage how cf sends mail. I think that will prove challenging. You'll see people suggest you perform cfmail in a loop with a pause (cf's sleep function). But that will take care to really manage the mail spool well. (Note the cf admin mail page setting for the current interval for cf to send mail found in the spool.)

Some might suggest how cfmail has a spoolenable attribute to disable use of the spool entirely: with that set to false, each such cfmail then is sent immediately, without piling up in the spool--and the cfmail awaits the mail getting to that smtp server - again, NOT ultimate TO address destination.) 

Finally, no, there's nothing to tweak in the admin to "slow down down its sending mail out" . There's only that mail spool interval, which defaults to 15 seconds, and that merely controls how often it wakes up to look in the spool for mail to deliver. YOU control in code how much mail you put IN that spool.

Again, you're best off seeking an alternate solution outside of cf (my first two points). Indeed, finally, someone will want to tell you "don't use cfmail for bulk email". They'd propose you give that job to a real mail delivery service (there are many, like mail chimp, constant contact, and more). Those don't suit everyone.

FWIW, I list both such mail services and alternate smtp mail servers (discussed earlier here) as categories in my cf411.com site.