[exim] TLS connection with Microsoft server stalls

Hi there,

sadly I didn’t make progress with my GMX problem. But during my tests I
came across another problem.
I’ve sent mails from @outlook.de to my server but the TLS encrypted
communication just stops at one point after some progress through EHLO,
MAIL FROM, RCPT TO (exim -bd -d+all):

10:04:45 15402 SMTP>> 250 Accepted
10:04:45 15402 tls_write(0x558a49151ac0, 14)
10:04:45 15402 gnutls_record_send(SSL, 0x558a49151ac0, 14)
10:04:45 15402 outbytes=14
10:04:45 15402 DSN: orcpt: NULL flags: 0
10:04:45 15402 Calling gnutls_record_recv(0x558a4920e660,
0x558a494e9000, 4096)

Then after 300 seconds the connection is closed.

This is with Exim 4.92, GnuTLS 3.6.7 from Debian Buster.
I already stripped away every config change I did for TLS ciphers or DH
params.

I know it worked in the past. And it still works without TLS. Maybe it’s
a problem of GnuTLS or maybe on Microsoft side, supposedly in some edge
scenario. But how can I debug it further?

Regards,
Christian