I had a similar problem that was addressed by Lotus support.
It seems that Groups cannot have more than one internet address. When I had two or more addresses in this field, no emails came through. Also, “Group names” cannot be email addresses with @s in them.
The work around for multiple group email addresses is to create a new user (no connected mail/nsf file required) as a proxy for the group. Put the internet address of the group in the “forwarding address” field, and put all aliases (or other email addresses) in the “User Name” field. I kept the “Mail system” as Notes. Now multiple variations of the group internet address deliver mail to the group members.
Lotus says that groups may be modified to have multiple aliases in future releases.