The mailto protocol is considered old school and may in fact be deprecated or disabled by default for some browsers mail clients. However, some users still find it useful. There are limits on how long a URL can be that varies with browsers and versions. Much of the information you find on the internet concerning mailto could be stale. One resource says Outlook has a limit of 1840 characters. So if you want to use mailto you pretty much have to experiment with different browsers and mail clients and accept the results.
That said if you are reaching the limit because of long recipient lists you could break this up into shorter recipient lists at the cost of manually sending more messages.
If you are reaching the limit because of long body text you could arrange for script to copy text to the clipboard and have the user paste the clipboard into the body.
Other possible solutions are to use some of the internal APIs QuickBase uses to send email under script control or to employ a proxy of some sort to send the email using a webservice or API.