Mac Mail is different as compared to Outlook and Thunderbird, and so are the settings. Most of the issues i’ve come across where sending stops working, is due to port and ssl settings. In a MAC, this setting can be easily adjusted from the Preferences. If you are using multiple accounts, make sure you don’t change the ports for the ones that are already working. When you EDIT the SMTP, or Create NEW (you will see the field) called In Use By which can help in identifying the e-mail account it is associated with. BUT if you only have one account, then just go ahead and do it.
Changing the SMTP Port on a Mac OS
Open the Mac Mail (by clicking on the mail icon from the dock). Once in the Mail, click Mail from the top bar
then select Preferences, and then Accounts. You will see the window like the one in the picture below.
Locate the Outgoing Mail Server (SMTP): tab and click on the up/down arrows. Then, select “Edit SMTP Server List“
This will open a new dialog, with two tabs (Advanced & Account Information). You can setup a custom port or set it to automatically detect settings, or check which port is being used. From the list of smtp servers, choose the server first before editing if you have more then 1 (probably) due to different e-mail accounts.
In the image above, i am using a custom port without letting the mail client automatically detect settings; this is because the provider has given me the settings; but if you do not know which port to use, then you can use the “Automatically detect and maintain account settings” option.