SMTP Error: Could not connect to SMTP host.

Whenever you get a connection error when sending via SMTP like

SMTP connect() failed.

The most common reason for this problem is that the ESP (Email service provider) you have defined in your delivery settings doesn’t accept one of the used email addresses.

Change the from-address, the reply-to, and the bounce address to the email you use to authenticate the SMTP connection.

If this doesn’t help Mailster is unable to contact the SMTP server you have specified in the Host property, but doesn’t say exactly why.

You can check connectivity by running some commands on your server (you will need dnsutils and telnet packages installed). First check that DNS is working:

dig +short smtp.gmail.com

You will get something like this if your DNS is working:

gmail-smtp-msa.l.google.com.
173.194.67.108
173.194.67.109

If this fails, Mailster will not be able to send emails because it won’t be able to obtain the correct IP address to connect to. If perhaps you don’t have a name in DNS, you can use an IP address directly as the hostname.

Next, try to telnet to the host on the port you need (this example is using TLS for Gmail):

telnet smtp.gmail.com 587

This should give you something like this:

Trying 173.194.67.109...
Connected to gmail-smtp-msa.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP ex2sm16805587wjd.30 - gsmtp

(Enter quit to get out of that). If port 587 doesn’t work, you can try port 465 or port 25, and use whichever one works – though bear in mind that port 25 doesn’t usually support encryption (see above).

If these connection attempts fail, Mailster will not work either. So go fix your network, then try again. If you are not in control of your own firewall or DNS, you probably need to raise a support ticket with your ISP to fix this. If they won’t fix it, you need to replace your ISP.