When sending html emails the best practice is to stick to old HTML3 and HTML4.
Email clients do not play well with CSS2 and up. Inline CSS work much better.
Instead of:
<a style="color: #FFFFFF; text-decoration: none" href="#/">
<span style="color: #FFFFFF; text- decoration: none">1800-000-0000</span>
</a>
By the way, you have syntax errors in this line, after each css declaration you need a ;
Corrected:
<a style="color: #FFFFFF; text-decoration: none;" href="#">
<span style="color: #FFFFFF; text- decoration: none;">1800-000-0000</span>
</a>
Try this:
<a color="#FFFFFF" text-decoration="none" href="#">
1800-000-0000
</a>
Do not forget to specify the HTML version on the DOCTYPE.
I've found myself working in both sides of this situation.
I worked on a web based email client using PHP with IMAP, I had to strip HTML emails
of a lot of things because they'd break not only the layout of the app but also
the intended behavior of buttons and forms. How? you might ask. With a desktop email
client this probably wouldn't be a problem but with a web based email client loading
external css/js files can result into a very large array of potential pitfalls and
nasty bugs.
On another occasion I was working on wedding invitations sent over HTMl emails,
The amount of inline css we had to do was ridiculous. To make it work on you have
to use mostly HTML3 and maybe a bit of HTML4 but not too much.
I recommend you to experiment with tables and deprecated HTML3 inline css.