Email delivery from boum.org to Schleuder lists @tails.net is broken

At the moment, if I send an email using, my boum.org account, to our Schleuder lists @tails.net, the message does not seem to be delivered. I've tried sending to 3 of those lists.

In our MTA VM it looks like for every such email I send, there's exactly 2 log lines in the Journal: connect quickly followed by disconnect. I'm providing 3 examples below:

Jul 25 14:10:23 mta postfix/smtpd[116753]: connect from confino.investici.org[93.190.126.19]
Jul 25 14:10:23 mta postfix/smtpd[116753]: disconnect from confino.investici.org[93.190.126.19] ehlo=1 starttls=1 quit=1 commands=3

Jul 25 14:10:47 mta postfix/smtpd[116753]: connect from latitanza.investici.org[82.94.249.234]
Jul 25 14:10:47 mta postfix/smtpd[116753]: disconnect from latitanza.investici.org[82.94.249.234] ehlo=1 starttls=1 quit=1 commands=3

Jul 25 14:11:28 mta postfix/smtpd[116753]: connect from devianza.investici.org[198.167.222.108]
Jul 25 14:11:28 mta postfix/smtpd[116753]: disconnect from devianza.investici.org[198.167.222.108] ehlo=1 starttls=1 quit=1 commands=3

While if I send an email using a Riseup account, it gets delivered, so perhaps the problem is specific to boum.org. I have no idea whether the problem is on Tails side, on boum.org side, or in the coordination between those.