I'm developing a RESTful API with Spring Boot as backend and an Angular 9 frontend. Upon user registration, a verification email is sent, in which the user needs to click on a link to verify their account. Functionally, all is set and working. However, the HTML is never rendered despite having Content-Type set to 'text/html' and charset to UTF-8. I'm using Thymeleaf to generate the HTML:
<!DOCTYPE html><html xmlns:th="http://www.thymeleaf.org"><head><meta http-equiv="Content-Type" content="text/html charset=UTF-8" /></head><body><header>TooManyThoughts</header><section><p th:text="#{registration.mail.greeting(${#messages.msg('mail.greeting.title.'+ user.personalData.title.representation, user.personalData.firstName, user.personalData.lastName)})}"></p><p th:text="#{registration.mail.preamble}"></p><p><a th:text="#{registration.mail.link}" th:href="@{http://localhost:8081/auth/register/email/verify/{id}/{key}(id = ${user.id}, key = ${user.emailValidationKey}, send='auth,login', verified='email')}"></a></p><p><span th:text="#{registration.mail.clue01(${user.credentials.username})}"></span><span th:text="#{registration.mail.clue02}"></span></p><p th:text="#{registration.mail.catchphrase}"></p><p th:text="#{registration.mail.goodbye}"></p><p><span th:text="#{registration.mail.signature01}"></span><br><span th:text="#{registration.mail.signature02}"></span></p></section><footer></footer></body>At the time of writing, all frameworks used are in their latest versions. Currently, I'm sending the email to my personal gmail account. When googling the issue, I've found some rendering issues with gmail, but they've all been old and from around 2011 or so. The only other posts and entries I could find were about setting Content-Type and charset properly, which in my opinion I've done.
I'm kinda stuck here since two days with what I believe should be a very basic issue, so any help is highly appreciated.