Connections to host SMTP on the defult ports

Depending on the provider, you will need to use TCP port 465 or TCP port 587.

Per Google: Incoming connections to the IMAP server at imap.gmail.com:993 and the POP server at pop.gmail.com:995 require SSL. The outgoing SMTP server, smtp.gmail.com, requires TLS. Use port 465, or port 587 if your client begins with plain text before issuing the STARTTLS command.

I’d try TCP 587 first, and enable connection security with whatever port you try.

Here’s the Apple mail settings lookup tool: Mail Settings Lookup

Here’s the Mac mail connection diagnostic tool: Use Connection Doctor in Mail on Mac - Apple Support

With High Sierra, you’ll have to use the Edit SMTP Server selection in the Mail Server pop-up within the Mail > Preferences > Accounts selection to get to the SMTP settings. It’s buried. There, you’ll find the account settings associated with sending to the mail server you’re working with. To smtp.gmail.com, in this case.

BDAqua:

TCP Port 25 is for mail servers to chat with other mail servers, and not a port that mail clients should be using. Not anymore. Not with common firewall rules intended to block spam. Many networks now block outbound TCP port 25 connections, except from known mail servers. Which will block client use of TCP port 25. Which means using TCP 465 and/or TCP 587 for sending mail.

The unencrypted ports (110, 143) should besr be avoided, unless your mail provider is operating without any security. Most providers do not support those insecure ports any more. And if the provider uses or requires those unencrypted ports, I’d look for a different provider.

The receiving email ports are:

 IMAP is port 143

IMAP-SSL is port 993

POP is port 110

POP-SSL is port 995

Outgoing ports are...

SMTP and SMTP-SSL is on ports 25, 587 and 465. Port 587 has to be SSL, and port 465 is enforced TLS-wrapped and is generally used by Outlook users.

Thanks. :)