"Mail delivery failed" emails being received today.

[#1]

Even though I haven't sent any emails for a number of days, I received 7 'spammy' emails in quick succession this morning 5 of which were "Mail delivery failed: returning message to sender" notifications and 2 were "Mail Delivery System: Undelivered Mail Returned to Sender" notifications. 

The first 5 each had 3 attachments (which I haven't opened) labelled as "details.txt", "untitled attachment 000085.txt" and an Outlook item titled "fabulous time". They also include different (but similar) 'recipient' email addresses which I don't recognise. The other 2 emails each had 2 attachments labelled as "details.txt" and "undelivered message header.txt". The sender's address of these 2 are shown as "MAILER-DAEMON@mail-proxy102.phy.heteml.jp" and "MAILER-DAEMON@smtp.at-system.net".

Can anyone advise me what's going on and what I need to do?  While typing this, another, similar  "Mail Delivery System" email has arrived!

[#2]

It somewhat sounds as though your email address is being spoofed.

Some toe-rag is sending spam emails using your email address. Delivery failures are then being returned to your email address. It happens all of the time and there is little which can be done about it whilst there are rouge email submission and transfer services which do not check to see if the use of a sending email address is legitimate.

Just delete them.

[#3]

Thanks Townman. Yes, I'm sure you're right. It was just that I hadn't seen anything like that before.

The bad news is that, to be on the safe side, I changed my Plusnet mailbox password and now I can't access my email via Outlook on my laptop. Outlook repeatedly asked for username and password when trying to send email. Spoke to the Plusnet helpdesk at length yesterday. As last resort, was advised to delete my mailbox and then add it back again. Currently am unable to add my account back as Outlook still doesn't accept my new password. Can access mailbox via Plusnet webmail site though. 

[#4]

Can you access anywhere on the internet?

Are you using a Plusnet router?

Is Outlook trying to access the default email address?

Which version of outlook?

[#5]

My internet access is ok.

I'm not using a Plusnet router. I previously did but moved home 4 years ago and my new home came with internet already installed. I just kept my Plusnet email account going. Everything worked fine until yesterday.

I have Outlook 2016. When I try to add my email account back, I enter the account name and password, incoming and outgoing server details (the Plusnet ones) etc and get as far as the test email being sent, and then I get a 550 error every time.