Bookstack cannot send e-mails

I’ve installed the linuxserver.io docker container for Bookstack. In the .env file I configured the smtp settings in order to enable the sending of e-mails. But when I try to send a test e-mail from Bookstack I am getting this error:
Error thrown when sending a test email: The BCMath functions must be enabled to use the NTLM authenticator.

I tried to install the php bcmath extension with a file in the “custom-cont-init.d” with the content:
apt update -y
apt install php-bcmath

But this did not change anything.
Are there any other Bookstack users with the same problem? How did you solve it?