VetBadger's email system uses industry-standard protocols to send HTML formatted email messages. All of the major email providers that we have tested are compatible with these messages. However, some older systems might not accept emails sent from VetBadger. If you have a client who works for a small company running their own email system, or who otherwise uses an out-dated email system, they might not receive the emails that you send.
The best solution for these situations is to ask your client if they also have a personal email address, provided by Google, Microsoft, Apple, Yahoo, or any of the other major email providers. Most people have an account with one of these providers, and if they don't then maybe it's time that they should.