Properly calling SMTP text sendMail multiple times

[mowmow65]

In trying to modify the "Sent_Text" code example for repeated calling, I'm having trouble properly restarting the session. I moved the sendMail code from setup() to a function called mailSend() and call that repeatedly from within loop(). But it appears that that I'm not properly reopening the session. If I begin malSend() with SMTPSession smtp;, then the mail results reporting looks like this:

20:43:34.100 ->
20:43:34.100 -> ----------------
20:43:34.100 -> Message sent success: 1
20:43:34.100 -> Message sent failed: 0
20:43:34.100 -> ----------------
20:43:34.100 -> {this sending result area is truly empty}
20:43:34.100 -> ----------------
20:43:34.100 ->

If I omit the SMTPSession smtp;, then I get a different extreme:

20:46:26.539 -> ----------------
20:46:26.539 -> Message sent success: 3
20:46:26.539 -> Message sent failed: 0
20:46:26.539 -> ----------------
20:46:26.539 ->
20:46:26.539 -> Message No: 1
20:46:26.539 -> Status: success
20:46:26.539 -> Date/Time: 2022/1/18 1:46:42
20:46:26.539 -> Recipient:
20:46:26.539 -> Subject:
20:46:26.539 -> Message No: 2
20:46:26.539 -> Status: success
20:46:26.539 -> Date/Time: 2022/1/18 1:46:57
20:46:26.539 -> Recipient:
20:46:26.539 -> Subject:
20:46:26.539 -> Message No: 3
20:46:26.539 -> Status: success
20:46:26.539 -> Date/Time: 2022/1/18 1:47:6
20:46:26.539 -> Recipient:
20:46:26.539 -> Subject: ESP32 Test Email
20:46:26.539 -> ----------------
20:46:26.539 ->

The above sending result is after sending three times (three calls to mailSend(). Note that the Recipient and Subject are garbage for all previous sends, and that only one email is sent per call.

Does another code example give a better clue on how to structure SMTP for more than one send?

[mobizt]

This is already comment.

ESP-Mail-Client/examples/SMTP/Send_Text/Send_Text.ino

Lines 236 to 237 in 633c98a

	//Because of pointer to const char that stores instead of dynamic string, the subject and recipients value can be
	//a garbage string (pointer points to undefind location) as SMTP_Message was declared as local variable or the value changed.

SMTPSession MUST be defined in global as it used widely.

[mobizt]

If you define SMTPSession as local, it will disposed when it exit that function scope and nothing keep in sending result.

SMTPSession hold the data and session (TCP) which can be reused, since first logged in.

Without third parameter for closing the session, the session will be closed as default.

ESP-Mail-Client/examples/SMTP/Send_Text/Send_Text.ino

Line 190 in 633c98a

if (!MailClient.sendMail(&smtp, &message))

When provide third parameter with false, the session will keep open (logged in)
MailClient.sendMail(&smtp, &message, false)

https://github.com/mobizt/ESP-Mail-Client/tree/master/src#sending-email-through-the-smtp-server

SMTPSession is not just a class to hold simple user data which you may think to used it freely but it reserved all session instance's resources which consumed large memory inside.

Any way, you can't define SMTPSession as local object as we need to keep it ready for data storing, TCP + SSL resources, and callback and this is the design of this library API, must define it globally.

[mowmow65]

Thank you!
I had seen the global requirement, so knew I had a problem, but what I missed was the commented-out "smtp.sendingResult.clear();", which did the trick.
Thanks again!

[mowmow65]

"When provide third parameter with false, the session will keep open (logged in)
MailClient.sendMail(&smtp, &message, false)"

By "logged in", do you mean literally logged into the SMTP server? If so, what is the best practice for sending one email per day? If I choose to end the session upon completion, how do I restart it?

[mobizt]

Session started when your device connected to server and it stopped when you logged out or internet interruption which both case the TCP connection will be closed by server and client respectively.

No matter how many message you want to send in the name of user of that mail server.

This is the same behavior as mail app, that you needed to logged in to read and send email, email was sent from user account email that logged in to use app.

Mail app may allows you to add other accounts to read and send mail which it keeps open the multiple sessions for that multiple accounts you added.

When session keep open after logged in, you will not need to send the log in credential to log in again.

In this library, when you want to change the account to another to send mail under the same SMTP server e.g. gmail.com, you need to logged out by closing the session.

How to close the session or log out? I already mentioned the third parameter of sendMail function above and the link to the function description, please take a look.

[mobizt]

You can defined multiple SMTPSession objets globally to open the sessions for multiple user account on different or the same SMTP servers which I'm not recommend because the SMTPSession objets reserved large memory inside for SSL/TLS resources for encryption/decryption process and leads to low free memory which caused the device crashed.

[mobizt]

To log in, just call

smtp.connect(&session);

Where session variable is the ESP_Mail_Session which hold the log in credentials.

[mobizt]

The ESP_Mail_Session and SMTP_Message can be define locally in the usage scope unless the garbage appeared in smtp.sendingResult that used the string from SMTP_Message that cannot access outside it defined scope.

[mowmow65]

Thank you again for the excellent explanation and for the fantastic email library.