Email configurations

Having problems with emails being sent from outlook via my server being put in junk from hotmail and yahoo and being rejected by gmail.

Gmail message for example:
host gmail-smtp-in.l.google.com[108.177.15.27]
said: 550-5.7.1 This message does not have authentication information or
fails to pass 550-5.7.1 authentication checks. To best protect our users
from spam, the 550-5.7.1 message has been blocked. Please visit 550-5.7.1
https://support.google.com/mail/answer/81126#authentication for more 550
5.7.1 information. l7si7325991wru.454 - gsmtp (in reply to end of DATA
command)

Our website has also stopped sending forms via the contact 7 wordpress plugin and needs confguring to work again.