[Ipv6] is not allowed to send mail from [my domain]

Some e-mail server is rejecting my e-mails:

host mail.xcellerate.nl [2a02:990:226:1:178:18:131:201]
SMTP error from remote mail server after RCPT TO:<xxx@xxx.com>:
550-[SPF] 2a01:7c8:fff8:3c:5054:ff:fea1:753e is not allowed to send mail from
550 mybox.net

2a01:7c8:fff8:3c:5054:ff:fea1:753e is not my ipv6 (my ipv6 passes SPF checks)

My box has an ipv6, which might be a problem? Is there something I can do on my end?

Have you tried to contact the administrator of the domain?

Have your tried https://dkimvalidator.com/ and https://en.internet.nl/?

(The second one should give you 3 ticks and one cross with a message about “Mail server connection not or insufficiently secured (STARTTLS and DANE)” - don’t worry about that last bit.