I tried this before … in all combinations, as suggested in another thread.
ssl-587, ssl-465, ssl-25
tls-587, tls-465, tls-25
none-25, none-465, none-587
I usually get the error “connection could not be established with host smtp.ionos.de [connection timed out #110]”
The funny thing: the nextcloud is hosted on Ionos/1and1-webspace as is the mail account … This should work in any way.