Mail problem

I use CDO.Message in my classic ASP application to send mail to my customers.

I create the body of the mail and always write links in it to link buttons to my website.

Sometimes links show wrong on customer's mails clients (Outlook, Gmail,....)

It is always the same routine I use to create mail body and links in it.

For example, 

str = " ... other HTML information ..."
urlDe = "https://www.domain.com/LinkRedir?LinkID=" & LinkID & "&LinkSID=" & LinkSID
str = str & "<p><a href=""" &  urlDe & """ target=""_blank"" ><img src=""https://www.domain.com/images/cab/vizaion.png"" border=""0"" align=""absmiddle"" /></a></p>"

str is the variable for the mail body.

The problem occurss when customer is reading the mail... the URL in the link has characters that I didn't wrote.. And It is always de %20 HTML char for space...

The mail has the image link with this URL :  https://www.domain.com/Link%20Redir?LinkID=SDGBsdfg9sah&LinkSID=D56

The problem is the %20 ... It appears in any part of the URL.. And not all my customers have the problem.

I don´t know why if it is always the same procedure with the same link structure, sometimes it is bad formed!

I really appreciate any help! Thanks!!!