Can't Send welcome mail. AuthenticationInvalidCredentials: 5.7.0 Denied

[sunrise]

My 3cx can't send Email. When I tested "SMTP server 3cx" have error: AuthenticationInvalidCredentials: 5.7.0 Denied

Then I change update server to https://downloads-global.3cx.com/downloads/v155
But problem not solved.
I use Free version and now can't change SMTP to another

Logs:
2018/02/22 02:55:03.612|2112|0021|Excpt|MailKit.Security.AuthenticationException: AuthenticationInvalidCredentials: 5.7.0 Denied
в MailKit.Net.Smtp.SmtpClient.Authenticate(Encoding encoding, ICredentials credentials, CancellationToken cancellationToken)
в MailKit.MailService.Authenticate(String userName, String password, CancellationToken cancellationToken)
в GenericUtilsNet.MailTemplateSender.Send(XDocument template, Func`2 getParameter, String mailTo, IEnumerable`1 attachments)
в _3cxSystemService.ExtSheduller.WelcomeEmailSender.SendWelcome(Extension ext)
в _3cxSystemService.ExtSheduller.WelcomeEmailSender.SendingProc()

 

[StefanW]

5.7.0 is a missmatch of your FQDN IP to the IP you are sending the eMail from. Check under Settings Network that the IP there is correct and your FQDN resolves accordingly.

 

[sunrise]

5.7.0 is a missmatch of your FQDN IP to the IP you are sending the eMail from. Check under Settings Network that the IP there is correct and your FQDN resolves accordingly.

What should I do if the Crimea is currently under sanctions and without VPN I can not update and send E-mail. But when connect via VPN, this error appears.

 

[StefanW]

you might set as public IP the VPN endpoints IP. Dependently if you have remote connected devices using the FQDN, then this would work.