Whenever I send an email, I get This is the mail system at host mail.[domain].
I’m sorry to have to inform you that your message could not
be delivered to one or more recipients. It’s attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<[email]@gmail.com>: host gmail-smtp-in.l.google.com[2a00:1450:4013:c06::1b]
said: 550-5.7.25 [2a02:c207:2273:8107::1] 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 match the sending IP. As a policy,
Gmail does not accept 550-5.7.25 messages from IPs with missing PTR
records. For more information, go 550-5.7.25 to 550-5.7.25
https://support.google.com/a?p=sender-guidelines-ip 550-5.7.25 To learn
more about Gmail requirements for bulk senders, visit 550 5.7.25
https://support.google.com/a?p=sender-guidelines.
4fb4d7f45d1cf-615a86e7182si4528112a12.332 - gsmtp (in reply to end of DATA
command) sent to me about 5 secs after