In my experience, the CSS suggested in the previous answers won't work across all email clients. The most failsafe way to ensure that long strings (i.e. URL links) will wrap within a container element is to dynamically insert <br> elements into a display version of the string. If a user copy & pastes the URL string into an address bar, the line breaks are ignored.

The following code is in Ampscript, however the principles remain the same for other languages.

VAR @link, @linkDisplay, @linkLen, @linkSegmentNo, @linkSegmentLen, @linkSub
 
SET @linkLen = length(@link)
SET @linkSegmentLen = 52
SET @linkSegmentNo = FormatNumber(subtract(divide(@linkLen, @linkSegmentLen), 0.5), "N0", "en-AU")

FOR @i = 0 TO @linkSegmentNo DO
 SET @linkSub = Substring(@link, Add(Multiply(@i, @linkSegmentLen), 1), @linkSegmentLen)
 SET @linkDisplay= Concat(Concat(@linkBreak, @linkSub), "<br>")
NEXT @i

<a href="@linkURL">@linkDisplay</a>

Where the @link is the original string and @linkDisplay is the string with <br> elements added at a preset segment length (the number of characters to include before adding the line break).