SMTP sending error: "5.7.8 Error: Authentication failed"

The following message appears when I try to send an e-mail through eM Client:

Of course, that is not my real e-mail, but it was informed at that format. I tried to inform it without informing the domain ("@sinos.net") but the same message appears anyway - and it works on Thunderbird without the domain part. Anyway, I do not believe that it is the cause of the trouble.

In fact, I’m facing this issue in 2 computers upgraded to eM Client 7.1 (latest version).

Here is the SMTP log:

09:46:52.379|01D|   SMTP S: 220 smtp.sinos.net ESMTP SINOSNET
09:46:52.379|01D|   SMTP C: EHLO [192.168.1.14]
09:46:52.476|01D|   SMTP S: 250-smtp3.sinos.net
09:46:52.477|01D|   SMTP S: 250-PIPELINING
09:46:52.477|01D|   SMTP S: 250-SIZE 38554432
09:46:52.477|01D|   SMTP S: 250-ETRN
09:46:52.477|01D|   SMTP S: 250-STARTTLS
09:46:52.477|01D|   SMTP S: 250-AUTH PLAIN LOGIN CRAM-MD5
09:46:52.477|01D|   SMTP S: 250-AUTH=PLAIN LOGIN CRAM-MD5
09:46:52.478|01D|   SMTP S: 250-ENHANCEDSTATUSCODES
09:46:52.478|01D|   SMTP S: 250-8BITMIME
09:46:52.487|01D|   SMTP S: 250 DSN
09:46:52.487|01D|   SMTP C: STARTTLS
09:46:52.590|01D|   SMTP S: 220 2.0.0 Ready to start TLS
09:46:52.812|01D|   SMTP C: EHLO [192.168.1.14]
09:46:52.903|01D|   SMTP S: 250-smtp3.sinos.net
09:46:52.903|01D|   SMTP S: 250-PIPELINING
09:46:52.903|01D|   SMTP S: 250-SIZE 38554432
09:46:52.903|01D|   SMTP S: 250-ETRN
09:46:52.903|01D|   SMTP S: 250-AUTH PLAIN LOGIN CRAM-MD5
09:46:52.903|01D|   SMTP S: 250-AUTH=PLAIN LOGIN CRAM-MD5
09:46:52.903|01D|   SMTP S: 250-ENHANCEDSTATUSCODES
09:46:52.904|01D|   SMTP S: 250-8BITMIME
09:46:52.904|01D|   SMTP S: 250 DSN
09:46:52.904|01D|   SMTP C: AUTH CRAM-MD5
09:46:52.963|01D|   SMTP S: 334 PDEyMTMzMTAzMDI0NDMyNzcuMTUwNzk4NTIxMUBzbXRwMy5zaW5vcy5uZXQ+
09:46:52.964|01D|   SMTP C: aXRuQHNpbm9zLm5ldCA3ODkzNDExYTdmM2E2YjdlOWI4Yjc0MGNhNjY1ODExMA==
09:46:54.510|01D|   SMTP S: 535 5.7.8 Error: authentication failed: PDEyMTMzMTAzMDI0NDMyNzcuMTUwNzk4NTIxMUBzbXRwMy5zaW5vcy5uZXQ+

Of course, my SMTP password is not wrong, because I’m using the same credencial for POP, and it is working normally.

Below is Mozilla Thunderbird log from the same mail provider:

[Main Thread]: I/SMTP SMTP Connecting to: smtp.sinos.net:587
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 220 smtp.sinos.net ESMTP SINOSNET
[Main Thread]: I/SMTP SMTP entering state: 14
[Main Thread]: I/SMTP SMTP Send: EHLO [192.168.1.8]
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-smtp3.sinos.net
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-PIPELINING
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-SIZE 38554432
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-ETRN
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-STARTTLS
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-AUTH PLAIN LOGIN CRAM-MD5
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-AUTH=PLAIN LOGIN CRAM-MD5
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-ENHANCEDSTATUSCODES
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-8BITMIME
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250 DSN
[Main Thread]: I/SMTP SMTP entering state: 4
[Main Thread]: I/SMTP SMTP entering state: 21
[Main Thread]: I/SMTP SMTP Send: STARTTLS
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 220 2.0.0 Ready to start TLS
[Main Thread]: I/SMTP SMTP entering state: 19
[Main Thread]: I/SMTP SMTP entering state: 14
[Main Thread]: I/SMTP SMTP Send: EHLO [192.168.1.8]
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-smtp3.sinos.net
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-PIPELINING
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-SIZE 38554432
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-ETRN
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-AUTH PLAIN LOGIN CRAM-MD5
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-AUTH=PLAIN LOGIN CRAM-MD5
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-ENHANCEDSTATUSCODES
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250-8BITMIME
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250 DSN
[Main Thread]: I/SMTP SMTP entering state: 4
[Main Thread]: I/SMTP SMTP entering state: 21
[Main Thread]: D/SMTP SMTP auth: server caps 0x22334, pref 0x300, failed 0x0, avail caps 0x300
[Main Thread]: D/SMTP (GSSAPI = 0x800, CRAM = 0x2000, NTLM = 0x4000, MSN =  0x8000, PLAIN = 0x200, LOGIN = 0x100, EXTERNAL = 0x400)
[Main Thread]: D/SMTP trying auth method 0x200
[Main Thread]: I/SMTP SMTP entering state: 16
[Main Thread]: D/SMTP SMTP AuthLoginStep1() for emclient@smtp.sinos.net
[Main Thread]: D/SMTP PLAIN auth
[Main Thread]: I/SMTP Logging suppressed for this command (it probably contained authentication information)
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 235 2.7.0 Authentication successful
[Main Thread]: I/SMTP SMTP entering state: 18
[Main Thread]: D/SMTP SMTP Login response, code 235
[Main Thread]: I/SMTP SMTP entering state: 3
[Main Thread]: I/SMTP SMTP Send: MAIL FROM:<emclient@sinos.net> BODY=8BITMIME SIZE=513
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250 2.1.0 Ok
[Main Thread]: I/SMTP SMTP entering state: 5
[Main Thread]: I/SMTP SMTP Send: RCPT TO:<rcpt@sinos.net>
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 250 2.1.5 Ok
[Main Thread]: I/SMTP SMTP entering state: 6
[Main Thread]: I/SMTP SMTP Send: DATA
[Main Thread]: I/SMTP SMTP entering state: 0
[Main Thread]: I/SMTP SMTP Response: 354 End data with .
[Main Thread]: I/SMTP SMTP entering state: 7

Of course, I removed confidential information from the log (only changed my e-mail and rcpt e-mail)

The Thunderbird log appears much more friendly: it appears to try to negotiate the authentication, eM Client apparently tries to forces it - maybe it is related to the eM Client configuration differs from Thunderbird (I couldn’t find STARTTLS on eM Client that my Thunderbird is using). But appears that eM Client fails on the handshake, before the authentication is send. But I’m not an expert on e-mail or criptography, so we need help.
Below there is eM Client POP3 log, that is working normally. Password is passed in clear text, so I removed it, and changed my email to emclient instead of the real one.

10:23:01.751|025| POP3 S: +OK mail.sinos.net Zimbra POP3 server ready
10:23:01.751|025| POP3 C: STLS
10:23:01.797|025| POP3 S: +OK Begin TLS negotiation
10:23:01.995|025| POP3 C: CAPA
10:23:02.067|025| POP3 S: +OK Capability list follows
10:23:02.068|025| POP3 C: USER emclient@sinos.net
10:23:02.119|025| POP3 S: +OK hello emclient@sinos.net, please enter your password
10:23:02.119|025| POP3 C: PASS Logging suppressed for this command (it probably contained authentication information)
10:23:02.171|025| POP3 S: +OK server ready
10:23:02.172|025| POP3 C: LIST
10:23:02.223|025| POP3 S: +OK 286 messages

10:23:02.356|025| POP3 S: .
10:23:02.361|025| POP3 C: RETR 286
10:23:02.420|025| POP3 S: +OK message follows
10:23:02.520|025| POP3 C: QUIT
10:23:02.564|025| POP3 S: +OK mail.sinos.net Zimbra POP3 server closing connection