sending email

I have problems with sending emails with my main server and with gmail. Neither of the two accounts show any willingness to send "Cannot connect to server". Any advice on what the problem is? My main service provider says it's the mac that's causing the problem. There's no problem with receiving...

Getting error on sending emails "...was rejected by the server because it does not allow relaying". iPhone = 4 iOS = 9.3.5 I have 4 accounts apparently on my phone: iCloud, Hotmail, and one related to my employer. Nearly everything related to email on my phone was done to me, not done by me...