Strange spam behaviour after full mailbox

I have the following situation,
The account of my mother in law receives alot of spam. For this she activated the payed service from her provider for spam-filtering.
She received about ten to 7-10 spam-mails per day.
Though easilly recognisable for spam by just looking at the mail-subject the spam-filter did not capture it, so the spam-mails in the normal inbox was still 7-10 per day and spam in the spam-box 4 per week.
The provider was unable to give any reason for this bad behaviour, and hide behind the fact that spammers got smarter and smarter.

Now comes the strange part.
They are on vacation, and since I maintain their IT and help them remove spam via the webmail, I checked the mailbox last sunday. Because they received many ppt-files and only have a 10 meg mailbox, they could't receive anymore e-mails. I checked and received an error back.
Now I have cleaned the mailbox, and checked again, it is not against the limit yet, but virtually no more spam-mails in the in-box (only one with a jpg-ad this week). The occasional spam-caught mail is still in the spam-box.
Are we experiencing a spammer who stopped spamming because he receives an error on mail-delivery? I thought they usually forge the reply-address and don't get answers on it.
What would be the most logical explanation for this?

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Some of the ISP's use 'smart' SPAM filters that can learn to recognize what actually is SPAM. In the tug-of-war, sometimes the good guys will start to catch up with the bad guys - for a while.
The spammer could have got caught and 'black-listed'.
However to answer your question, "Are we experiencing a spammer who stopped spamming because he receives an error on mail-delivery?" - I would say the answer is no way.
Regardless of why it has 'paused' (not stopped) the spammer is making too much money to stop now. Further, even if he stops, there are about 14 more behind him to take his place.
I would just be grateful that it has paused and pretend (to your mother-in-law) that you deserve all the credit.
Until it starts up again.
Good luck on this - it is a pain for everyone.
maduropaAuthor Commented:
@jvuz,  I have none of them spams left, they were very typical. If we receive one back, I will check if it is a correctly receiving account,

@younghv, unfortunatley, the ISP is not very good (and/or smart) at catching spam-mails. I'll not repeat the words, but they are plain in the e-mail, but the filter doesn't get them.  I indeed do fear that it is not defintive stopped, so I fear taking credit, and it might continue again.

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