Centos8 Thunderbird no outgoing connection - centos

Question

i face the following problem.
OS: CentOS 8.1.1911
Mail Client: thunderbird-68.7.0-1.el8_1.x86_64
Mail Provider: gmx.net
If i try to sent an mail the thunderbird doesn't connect to the network interface. Telnet does.
telnet mail.gmx.net 465
Trying 212.227.17.190...
Connected to mail.gmx.net.
Escape character is '^]'.
^CConnection closed by foreign host.
So, firewall is open on my computer and in the router.
With wireshark i can see the connection from telnet goes over my internet connection. But thunderbird doesn't show up there.
With strace i face a problem, in a loop:
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
poll([{fd=4, events=POLLIN}, {fd=5, events=POLLIN}, {fd=10, events=POLLIN}, {fd=34, events=POLLIN}], 4, 0) = 0 (Timeout)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
poll([{fd=4, events=POLLIN}, {fd=5, events=POLLIN}, {fd=10, events=POLLIN}, {fd=34, events=POLLIN}], 4, 0) = 0 (Timeout)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
poll([{fd=4, events=POLLIN}, {fd=5, events=POLLIN}, {fd=10, events=POLLIN}, {fd=34, events=POLLIN}], 4, 0) = 0 (Timeout)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
poll([{fd=4, events=POLLIN}, {fd=5, events=POLLIN}, {fd=10, events=POLLIN}, {fd=34, events=POLLIN}], 4, 0) = 0 (Timeout)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
recvmsg(4, {msg_namelen=0}, 0) = -1 EAGAIN (Die Ressource ist zur Zeit nicht verfügbar)
poll([{fd=4, events=POLLIN}, {fd=5, events=POLLIN}, {fd=10, events=POLLIN}, {fd=34, events=POLLIN}], 4, -1^C) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
strace: Process 103995 detached
How to proceed?
Where can i check further why thunderbird doesn't connect to my internet connection?
Any advise is welcome.
BR Dirk