If I see an email address in the TO field of an email that I received, does that mean it was definitely sent to that address, too? [closed]

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This is probably a stupid question, but...

I've got an application that sends emails out, programmatically. The emails have two recipients. One in the TO field (my client), and one in the BCC field (me).

I'm getting the emails, and I see my client's address in the TO field, but they sweat they aren't getting them. Does the fact that their email is showing 开发者_Python百科up in the TO field at least mean it correctly left my server?

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No, it doesn't. The To is only a header. For instance, try to email xalalaa@aeihiaghjaerhgkjahegh.com, which I suppose is non-existant, and put yourself on BCC. You will receive the email

Probable causes:
1. You are on the same domain, so the mail delivery subsystem can find you, but has issues mailing others (if this is the case, try to BCC an external mail of yours)
2. If you are not on the same domain, there are plenty of possibilities, check every customer and whether no one has received the message.

EDIT: Quoting Mike Daniels too:

Regardless, if there is a failure, I'd expect to see a message from your (or their) e-mail server indicating that the message couldn't be delivered.

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E-mail headers are not reliable. In fact, it's possible to create an e-mail message that appears to come from a completely different person, just by modifying the header. (As an aside, this is one reason why digital certificates are useful.) The presence of your recipient in the "to" field is no guarantee of anything at all. Regardless, if there is a failure, I'd expect to see a message from your (or their) e-mail server indicating that the message couldn't be delivered.

Edit: New thought. Is it possible their e-mail server is dropping the message silently because it's coming in with some weird header? I am no e-mail server expert, but they could be dropping messages that have no sender or reply-to address in the header.

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It should as long as it's the right e-mail address. Have they checked their spam filter?

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Others have mentioned that you should be seeing an error reply if the message can't be sent. But that assumes that the emails have a proper "From" or "Reply-To" header. Try replying to one of the messages that you get via BCC, and see if it goes anywhere.