This advice is mostly for Outlook, but the below is good practice for all email clients to ensure your subject line does not get cut off.

When using variables inside a mailto link, you must ensure there are no special characters in the subject parameter, such as '&'. If there is a special character such as this, you will see the subject get cut off at the special character when Outlook processes this link. 

Say you have a variable called 'event-name' which contains the string 'Dinner & Drinks', if you use this 'event-name' variable in a mailto link, it will simply appear as 'Dinner' in the subject line in Outlook.

To avoid this, you should make a separate variable named something along the lines of 'event-name-mailto' and replace the special characters with '%' followed by their hexadecimal value.

See below for examples of a 'Contact Us' link with hexadecimal Ampersand (%26) and a 'Contact Us' link with the special character Ampersand '&'.

Using hexadecimal values for the special character (feel free to set default email app to Outlook and test these links):

• Contact Us (Using %26 in place of '&', this will populate the subject line in Outlook correctly)
  URL: mailto:test@test.com?subject=Dinner %26 Drinks

Using the special character:

• Contact Us (This simply has '&' between the 'Dinner' and the 'Drinks', this gets cut off due to Outlook not knowing what to do with the special character)
  URL: mailto:test@test.com?subject=Dinner & Drinks