There seems to be an update that has run which a few mac mail clients have run into.
The simplest fix at the moment is to change your incomming and outgoing settings to your hostname if you ping your domain name in terminal this will give you your server name.
To do this at the top right hand corner by your finder type the word termial in this will open your terminal.
Once open type ping yourdomainname.co.za
Once you have this log into mac mail change your incomming server and outgoing server to the output of your ping.
PING maxxess.co.za (18.104.22.168) 56(84) bytes of data.
64 bytes from martel.aserv.co.za (22.214.171.124): icmp_seq=1 ttl=55 time=3.78 ms
64 bytes from martel.aserv.co.za (126.96.36.199): icmp_seq=2 ttl=55 time=2.89 ms
--- maxxess.co.za ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 2.892/3.338/3.785/0.450 ms
martel.aserv.co.za will be my incomming and outgoing setting with use ssl on and 993 or 995 for incomming depending if its pop or imap.
And 465 for outgoing.
if there is anything else you need please let me know.