Im using SMTP relay as there are no limits on it, the API has a throttle of 1000 emails at a time I think.
Try leaving the port number default at 25 to test out.
Your username should just be : apikey
and nothing else. For the password, It sounds like you’re maybe using the ‘API Key ID’ which is just an internal identifier. You need to use the apikey value itself as the password.
Frustratingly, you’ll only be able to see the full apikey once after you create it, so if you didn’t keep a copy of it, you’ll need to generate a new key.
Here’s what SendGrid say about it:
You will only be shown your API key one time. Please store it somewhere safe as we will not be able to retrieve or restore it.
There’s help here:
Post screenshots of your set up if you are still having issues.