We can see that xxxxxxx@xxxx record is marked as Email Invalid with cause '550 5.4.1 Recipient address rejected', and unable to send any email from Marketo.
But Salesperson is able to send him email outside of Marketo.
Can anyone please let me know when the 550 5.4.1 error occur and why Salesperson is able to send email outside of Marketo?
Ah yes, that's actually quite a common error message, but also a very unsatisfactory one because it doesn't tell you a whole lot. The one thing you do know is that the recipient's email server does not accept emails from your domain. The reason why however could be any of quite a few.
I find this article on the Microsoft docs site quite helpful in understanding it: https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange-....
I deal with 550 bounces almost on a daily basis and we have attempted to call customers back to confirm that the email address is correct and in some cases there is no problem.
I would recommend to use the services of a data cleanup company, like Fresh Address, that scans your email addresses and lets you know if your email addresses are safe to send. This way you can prevent bounces ahead of time without compromising your reputation among ESP's. Another suggestion if you don't have that many leads and can manage this is to have someone call them not just to validate the email but perhaps to see if they have another email as a backup that you can add to their account.
I would recommend to use the services of a data cleanup company, like Fresh Address, that scans your email addresses...
Well, yes and no.... when email validation services run their checks, the SMTP connection doesn't originate from the Marketo server farm. Nor does it have the standard Marketo identifying marks — envelope sender pattern, DKIM signature, etc.
So such services can't check the person's emailability from Marketo, as that's something only Marketo can do. And some recipients focus specifically on blocking mail from marketing automation apps/platforms.
(I've built an email validation service in the past, so when I talk about such gaps it's a matter of full disclosure.)