By default when you send an email through frappe.sendmail() By default value in that function is delayed=True
For send an email instantly without being in queue you can just pass delayed=False
frappe.sendmail(
recipients=recipients,
subject=frappe._('Birthday Reminder'),
template='birthday_reminder',
args=dict(
reminder_text=reminder_text,
birthday_persons=birthday_persons,
message=message,
),
header=_('Birthday Reminder'),
delayed=False
)