SIM800C GSM Module: Sending Email via SMTP on Port 587 Without SSL

Question

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I did a test of the GSM module SIM800C , which I used to send an email using the SMTP protocol. During the test, I sent AT commands to the module via the YAT terminal, and the whole process can be seen below. To summarise, I will write that I sent the email using the outgoing mail server smtp.poczta.onet.pl on port 587 (without SSL encryption).

Connection diagram.

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AT commands sent.

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krzbor · Answer

It is difficult to say what is wrong, but it could be that only encrypted transmission is allowed on port 587 i.e. STARTTLS is required. There is a dialogue going on with the mail server i.e., every message sent has a response - see if there are no AT commands to see these responses - "OK" is not a server response but rather a confirmation that something has been sent. On Windows you can install telnet (it's in the additional applications) you can then issue the command:
telnet smtp.poczta.onet.pl 587
and talk to the server (read about the SMTP protocol to know how to talk)

czasnagli · Answer

. This topic is not a description of the problem, but a working example of how to send an email using SMTP. Port 587 is for connections without SSL encryption. . Confirmation that an email has been sent is the response +SMTPSEND: 1 . .

krzbor · Answer

For Gmail:
TLS/STARTTLS connection:
Outgoing mail server - smtp.gmail.com - port - 587
Port 587 is usually for encrypted and unencrypted transmission. The start is always unencrypted, but once the connection is established there may be STARTTLS and a switch to encryption. Of course, onet may work differently.
Have you checked the response (dispatch confirmation) if you enter intentionally wrong authorisation data? Is it also always "OK"?

czasnagli · Answer

. If I enter incorrect authorisation data then the module returns OK , followed by +SMTPSEND: 67 .

krzbor · Answer

That is, communication and authorisation is OK. Rather, the SMTPSEND result can also be trusted. This leaves the problem of the SMTP server rejecting such a message. Servers can verify the FROM with the real address (account) - in your case, however, there is a match. Maybe it doesn't like sending to itself? If you have access to another SMTP server then you can check another server.

czasnagli · Answer

. The SMTP server did not reject the message because I read it on Onet mail. I wrote in the first post that the email was sent successfully. I repeat again that this topic is not a description of the problem.

krzbor · Answer

And I thought you were sending, you have the correct status and the mail is not coming .

czasnagli · Answer

. Everything is working properly. I would still add that the module described above only supports connections without SSL.

SIM800C GSM Module: Sending Email via SMTP on Port 587 Without SSL

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