Riseup rejecting my friends messages? Or something else going on?

S a posé une question...
Hiya

my colleague who uses aol, for the last few weeks is being told (but only every few times she emails me, not every time) that her message can't be delivered to me. See below error message that she recieves. Any idea?
thank you!
S

-----Original Message-----
From: MAILER-DAEMON
To: birthyorkstorks
Sent: Wed, 17 Jul 2019 22:59
Subject: Failure Notice

Sorry, we were unable to deliver your message to the following address.

:
550: 5.7.1 Service unavailable

--- Below this line is a copy of the message.

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=aol.com; s=a2048; t=1563400733; bh=oEykbKe+AndX2lllxxBYTi8zXxU7lE4O4688IFBavCU=; h=Date:From:To:Cc:Subject:References:From:Subject; b=h4DAttl+gncbFzY4IjTrGjm8pLGlNZYH3KiZqhJQX5XcQaMHNUhRSw5LViX3FPjEtQXut0VXndrO2EV6i/d6YwuVuJVZXSltPaXMJSjV4rA+VEH+JZBVACkxUKBAuA7AmgGQwraIS09Vg/JmZYd/8sq9PA9r+u5LG1q0j2oYunoivH8jCi4Jr5hx+I5xgxJiJjBit1XeK50uWRNCtN0QaqqT/ozeHSaLmVtJ2vKTDdqEsc6nwydi0q4lnKeJ4hunRgYPpTcN6NHm4fam53U8s6hJizd7ypQmodd+/jUJHzHHlnOapEd3vMzwa5Yj9DAzBgA102gzSnXs0RJAI6Bm0w==
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Date: Wed, 17 Jul 2019 21:58:50 +0000 (UTC)
From: birthyorkstorks@aol.com
To: annamactobin@gmail.com
Cc: claireharbottle@googlemail.com, oxytocin@riseup.net, debsagos@yahoo.com,
cosmickitten_17@hotmail.com
Message-ID: <2019870997.1683370.1563400730491@mail.yahoo.com>
Subject: Twins near Hull - another twin mum
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References: <2019870997.1683370.1563400730491.ref@mail.yahoo.com>
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