I have configured Email-to-case and when I send an email from [email protected](for example) to the service address the case is not getting created but when I send an email from [email protected] then case is getting created.
So why it is working for one email address and not for others?
Do you send to some friendly mailbox like "[email protected]" or to the long unreadable SF address like "[email protected]"? Maybe the forwarding rule on your mail server (if any) forwards only certain messages.
What are your email-to-case settings. Maybe you allow only plaintext emails and no attachments and the 2nd message is sent as html or with attachments.
At the bottom of the Email-to-Case page there will be routing address (like my "[email protected]"). In there there's tiny little field about "accept email from". Either keep the field blank or make sure both addresses are added?
Do you use standard Email-to-Case (basic stuff) or do you have an Apex class as inbound email handler? If Apex - you can set debug logging on the running user and see if it fails in code, on validation rule during insert or what.
EDIT, more things to check
Interesting.
Do you send both to same address? If to different ones and they're configured with diff. case record type - check in Setup -> Support Settings if Automated Case User has permission to all record types. If you have different record types and validation rules that depend on type and they can't pass if field isn't populated from email - that might be a problem
Did you get any error emails in that mailbox from [email protected]? They might be in junk/spam folder
Can you put debug logging on that automated case user and see if anything pops up
Does the email have something that would fail to save like subject > 255 characters?
Go to Setup -> Monitor -> Email Snapshots and request a snapshot, maybe there's something interesting in there, maybe it doesn't arrive in SF at all and then you need to check the email server, forwarding rules...
Is it on Gmail? There might be an extra step in config, see https://help.salesforce.com/articleView?id=000328923&type=1&mode=1