Some email accounts are unable to send email to gmail. The error email is
The mail system
gmail-smtp-in.l.google.com[2a00:1450:4010:c05::1b] said: 550-5.7.25
[2a01:4f9:4a:4341::2] The IP address sending this message does not
550-5.7.25 have a PTR record setup, or the corresponding forward DNS entry
does 550-5.7.25 not point to the sending IP. As a policy, Gmail does not
accept 550-5.7.25 messages from IPs with missing PTR records. Please visit
550-5.7.25 https://support.google.com/mail/answer/81126#ip-practices for
more 550 5.7.25 information.
l21-20020ac24315000000b004fbb6cae549si3022921lfh.296 - gsmtp (in reply to
end of DATA command)
This happens with some email accounts, other accounts successfully send email to gmail.