You'll have to look to Instagram support for their advice on accounts that may have been hacked.
Superuser 2017/18 Use Kudos to say thanks Mark answer as "helpful" only when the problem is solved Please don't send me private messages unless I ask you to. I do not work for VM. The advice I give is based on my best understanding of VM policy and practice. You rely on it at your own risk.
I'd concur with Howard, rather than assuming it's just spoofing, if it were my account, I'd definitely follow the steps in the link Howard's provided. ESPECIALLY as their Instagram account has been hacked also.
If this were my account and I were getting mailer-daemon messages for mails I hadn't sent - The first thing I would do is change my password AND security question and answer over at my.virginmedia.com.
As Howard has pointed out the from address on the bounce messages can be a good indicator of hacking vs spoofing. If the from address is mailer-daemon<at>virginmedia.com - then it's a good bet your password has been hacked. Make sure you use a strong password with a random mix of upper and lower case letters and numbers.
Consider investing time in setting up a password manager such as LastPass or Dashlane (to name but two) - secure them with two factor authentication and use them to set separate secure passwords on your accounts.
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