member cannot send to members-XXXX@toastmastersclubs.org

When our member who uses email address YYYY@proton.me tries to send an email to our club members using members-5208@toastmastersclubs.org, her email bounces with the attached message.  Key part I think:
X-Spam-Status: No, score=-5.0 required=5.0 tests=HTML_MESSAGE,RCVD_IN_DNSWL_HI,
    RCVD_IN_MSPIKE_H5,RCVD_IN_MSPIKE_WL,RCVD_IN_ZEN_BLOCKED_OPENDNS,
    SPF_HELO_PASS,SPF_PASS,T_SCC_BODY_TEXT_LINE,URIBL_BLOCKED,
    URIBL_DBL_BLOCKED_OPENDNS,URIBL_ZEN_BLOCKED_OPENDNS
    autolearn=unavailable autolearn_force=no version=3.4.3
X-Spam-Checker-Version: SpamAssassin 3.4.3 (2019-12-06) on
    toastmastersclubs.org

Is this toastmastersclubs.org's spamassassin rejecting proton mail?  Or proton mail's spamassasin rejecting toastmastersclubs.org?
What can we do about this please?

Thank you.
Cheers
Cliff Hall
Club 5208 Auckland West Toastmasters Club

Need at least the member's first name for troubleshooting

Hi Steve.  The member's first name is Nike.  So her email address is nike@proton.me.
Cheers
Cliff

That email address is not the email address that is entered for them in Membership Management.  Please take a look.

Hi Steve
You are correct, sorry.  Her email address is nikefranke@proton.me
That is the email address she is having trouble sending from.  In other words, the email address she is using matches the one in membership management.  It is my reporting in this thread that was wrong.  Sorry for the confusion.  You can see the email address used in the attachment.
Cheers
Cliff

I can see many emails going to Nike, but only 1 email from her 3 weeks ago.

Please ensure that she is emailing to the correct email address, as listed on your website under the menu "Club E-mail addresses"