Each time the RETR command is issued your server directly drop the connection. You can see that again and again in the connection log. RETR means RETRIEVE and is the POP command to fetch an email, the parameter is the email number. RETR 1 means RETRIEVE mail number 1. The normal response or the RETR command is the message contents, not a connection drop.

Outlook/Thunderbird is able to get the email successfully via the same server.

Below is the log from mail server using bounce program.
May 8 11:17:56 xxxx dovecot: pop3-login: Login: user=<xxxx>, method=PLAIN, rip=xxx.xxx.xxx.xxx, lip=xxx.xxx.xxx.xxx, TLS
May 8 11:17:56 xxxx dovecot: POP3(xxxx): Connection closed top=0/0, retr=1/38460, del=0/1401, size=31687200

Have you tried not use TLS?

I've already tried another mail server.
No problem without TLS.
But this mail server must use TLS.

The server then has a problem with TLS reconnection. What interval are you using?

What interval value do you mean?
Where can I check it?

In the Settings panel, 'Check Mailbox Every xxxxx Minutes'. What is your selection? Have you tried to change it?

Any fix for the Gmail problem? I will switch to another webmail for the next sending, but need to extract this mails first.

Thanks,
S@

Well, I can't do nothing on Gmail servers. Gmail is not suited to bulk emailing and Google will make everything to make it work as badly as possible. My advice, run away from Gmail!

I use workaround: Thunderbird -> Save selected messages: as txt format -> Email Extractor.

This did the job also