gnus-outgoing-message-group

gnus-outgoing-message-group is a variable defined in `gnus-msg.el'.
Its value is

nil

• This variable is obsolete since 24.1;
  use `gnus-message-archive-group' instead.

Documentation:
All outgoing messages will be put in this group.
If you want to store all your outgoing mail and articles in the group
"nnml:archive", you set this variable to that value. This variable
can also be a list of group names.

If you want to have greater control over what group to put each
message in, you can set this variable to a function that checks the
current newsgroup name and then returns a suitable group name (or list
of names).

You can customize this variable.