All my registration pages successfully send clickable links via email (using Symfony). However, I have a mail opt in page that's has similar code, but the links aren't turning up as clickable.

I've now set up identical test emails on the mail optin php page and one of my registration pages, and am sending just the following in each email:

$body = "https://www.blahblahsite.com"

The sender, populated with the same code for each test, is:

if (!$result = AppEmail::sendEmail($to, $subject, $body, $from, null, null))
{
     //error handler
}

I've checked that my test code on each page is the same, yet one link still turns up not clickable.

The other factor is that I use gmail to send the emails:

$transport = Transport::fromDsn('smtp://myemail@blahblahsite.com:password_here@smtp.gmail.com:587?encryption=tls&auth_mode=login');

What other factors can determine whether a link arrives clickable please?

Thank you.