Sender uses our email as sender

Hello,

Our customer uses Sophos Central Email.

Unfortunately, he constantly receives emails from himself (fake sender).

How can we stop this from happening?

"
To: info@****
Subject: There is an overdue payment under your name. Please,
settle your debts ASAP.
Content-Type: text/plain; charset=ISO-8859-2; format=flowed
Content-Transfer-Encoding: 8bit
X-Sophos-MH-Mail-Info-Key: ******************************
X-HE-Spam-Level: ++++++++++++
X-HE-Spam-Score: 12.3
X-HE-Spam-Report: Content analysis details: (12.3 points)
pts rule name description
---- ---------------------- --------------------------------------------------
2.4 DATE_IN_FUTURE_03_06 Date: is 3 to 6 hours after Received: date
0.4 RDNS_DYNAMIC Delivered to internal network by host with
dynamic-looking rDNS
1.0 BITCOIN_EXTORT_01 Extortion spam, pay via BitCoin
0.0 PDS_RDNS_DYNAMIC_FP RDNS_DYNAMIC with FP steps
0.4 PDS_BTC_ID FP reduced Bitcoin ID
3.2 HELO_DYNAMIC_IPADDR Relay HELO'd using suspicious hostname (IP
addr 1)
3.5 BITCOIN_SPAM_07 BitCoin spam pattern 07
1.4 BITCOIN_SPAM_02 BitCoin spam pattern 02
X-SPAM-FLAG: Yes
X-HE-SPF: FAILED
"