mailbox bcc recipient addresses malformed

What is happening:

If entering more than one E-Mail Address into the field: ISPConfig -> Email -> Email Mailbox -> Send outgoing BCC to: (seperate multiple email addresses with comma)

the addresses are not handled correctly (eg: mail@example1.com,mail@example2.com is set as <mail@example1.com,mail@example2.com> recipient) This address is unknown and rejected my the - in this case example2.com - receiving mailserver. This happens also with an additional space between the addresses (eg: mail@example1.com, mail@example2.com is set as <mail@example1.com, mail@example2.com>)

correct behaviour

Mail should be sent to both (or more) email addresses

environment

Server OS: debian Server OS version: jessie ISPConfig version: 3.1.13p1

references

This has also been discussed here: https://www.howtoforge.com/community/threads/sender_cc-merge-mail-address-wrongly.79271/