Bad query performance with more conditions?

Hi All,

I have a table named emails which created by following script:
create table emails (
    email_id bigserial primary key,
    email_msg_id char(36) not null,
    email_sender text not null)

The table has more than 10 million of records and I have already
created indexes on email_msg_id and email_sender

Following script finished very quickly (less than 1 seconds):
    select * from emails where email_msg_id =
'4dba381b-f55e-02d3-4b4a-95e2e98178e2' limit 1
But a similar script cost more than ten seconds:
    select * from emails where email_msg_id =
'4dba381b-f55e-02d3-4b4a-95e2e98178e2' and email_sender_text =
'sender@domain.com' limit 1

Both those scripts returns 0 rows of records indeed.

Any comment are highly appreciated.