Cannot send email if the main address is redirecting to gmail

[João Garcias]

Hello Everyone, 

I've an issue which i cannot see what is happening, If I send an email  to a gmail account, it works perfectly. but if I send an email to a custom address and that specific address is diverting to gmail. I receive this message below.  

email address removed for privacy reasons
Remote Server returned '550-5.7.26 This message does not pass authentication checks (SPF and DKIM both 550-5.7.26 do not pass). SPF check for [mydomain.com] does not pass with ip: 550-5.7.26 [2a01:XXX:fXX0:fe13::202].To best protect our users from spam, the 550-5.7.26 message has been blocked. Please visit 550-5.7.26 https://support.google.com/mail/answer/81126#authentication for more 550 5.7.26 information. v18-20020a17090610d245c668415si19935811ejv.687 - gsmtp'

example:  I sent an email  to Xpt @domain and  this email is is diverting to gmail mail 

my spf is correct. 

[#2]

Is it to a contact or with mailforwarding?

Exchange version?
Maybe some more information would help

[#3]

There's not really a lot you can do once the messages leaves your Exchange infrastructure and is delivered to the intended recipient. If that recipient has some method of forwarding messages, then it is entirely possible they will get these types of issues...that's just how message authentication works (forwarding can be a method hackers user to get obtain data).

[#4]

it's a mail forwarding and we are using exchange 2016 version 15.1 (2308.8) , I could replicate this. I have an hotmail account which is forwarding to a gmail account, if I send an email to gmail works fine, if I send an email to hotmail account I got this specific error.

@gmail
Remote Server returned '550-5.7.26 This message does not pass authentication checks (SPF and DKIM both 550-5.7.26 do not pass). SPF check for [DOMAIN.COM] does not pass with ip: 550-5.7.26 [].To best protect our users from spam, the 550-5.7.26 message has been blocked. Please visit 550-5.7.26 for more 550 5.7.26 information. v18-20020a17090610d200b008845c668415si19935811ejv.687 - gsmtp'