Not connect to SMTP

[glowndark]

I have a problem with sending e-mail from the mail client. In e-mail client I have about 5 accounts are automatically checked every 10 minutes. e-mails can be obtained from mail client, but when trying to send out a message not connect to SMTP server. This error sending appear only rarely. (not a problem with the internet). The rest of the time everything works ... Where could be the problem?

[cPanelDon]

Please ensure that SMTP authentication is enabled for each configured e-mail account within the mail client software; this is where your mail client authenticates using an e-mail account username and password when sending via SMTP.

[glowndark]

Please ensure that SMTP authentication is enabled for each configured e-mail account within the mail client software; this is where your mail client authenticates using an e-mail account username and password when sending via SMTP.

As I said in the beginning, when start the mail client everything works, I can send from all accounts, so the problem is not with autorization of SMTP.
Problem occurs after 1-2 hours. Then from any of the accounts can not send e-mail. I tried after the problem occurs change my IP (it's dynamic) occur after change IP again I can send e-mail.
So I think for some reason after a certain time, IP is blocked from my server and so I can not send e-mails. Mail client is set to check for new mails every 10 minutes I did not send many mails, 2-3 per hour ...

[InstaCarma_Tech]

It seems that the IP is getting blocked in the server firewall. You can find out the reason from the logs. If you do not have access to them then contact your host. They will be able to tell you the exact reason and resolve the issue.
Btw, which firewall is being used on the server?

[votethehost.com]

There is option in csf firewall to block the IP on failure logins. I think one or some of your configured mail account password is wrong Every 10 mins your email client is trying to login to server with that wrong password. After few failure logins firewall is blocking you for 30 mins (by default) and then your IP is released.

I would suggest you to recheck that you have set correct password for ALL mail accounts.