Major versions of Yii Mailer and Yii Mailer Symfony Mailer packages were tagged.
The Yii Mailer abstraction for sending emails has been significantly reworked.
Now, the package yiisoft/mailer is independent of any other dependencies and essentially consists of two interfaces: MessageInterface (a message with methods for setting/getting values) and MailerInterface (a service for sending messages with two methods: send() and sendMultiple()).
Yii Mailer changes:

• Add NullMailer and StubMailer.
• Add FileMailer configuration.
• Add HtmlToTextBodyConverter.
• Add MessageInterface methods: getAttachments(), getEmbeddings(), getHeaders(), withAddedAttachments(), withAddedEmbeddings().
• Add Message class that implements MessageInterface.
• Remove MessageFactoryInterface and MessageFactory, add MessageSettings instead it.
• Remove MessageBodyRenderer and MessageBodyTemplate classes.
• Remove yiisoft/view dependency.
• Remove MessageInterface methods compose(), withTemplate() and withLocale().
• Rename Mailer to BaseMailer, remove beforeSend() and afterSend() methods, finalize methods, and mark dependencies as read only.
• Remove getMessage() method from AfterSend and BeforeSend classes, make $message property public read only instead it.
• Change result type of MessageInterface::with* methods from self to static.
• Remove implementation of StoppableEventInterface from BeforeSend and add $preventSendingMessage property to it.
• Remove beforeSend() and afterSend() messages from BaseMailer.
• Rename MessageInterface methods: withEmbedded() to withEmbeddings(), withAttached() to withAttachments() and allow passing several files to them.
• Remove getError() and withError() methods from MessageInterface.
• Change result of MailerInterface::sendMultiple() to SendResults object.
• Use new Priority enumeration instead of integer value for define priority in message.
• Change order of constructor parameters in Mailer and FailMailer.
• Make psr/event-dispatcher dependency optional.
• Replace RuntimeException to LogicException when file name callback of FileMailer returns non-string value and improve exception message.

Yii Mailer Symfony Mailer Extension changes:

• Change package configuration params prefix to yiisoft/mailer-symfony.
• Adapt to Yii Mailer 6: remove Message class, add $messageSettings parameter to Mailer constructor, remove MessageBodyRenderer usage.
• Raise minimal PHP version to ^8.1.
• Change order of constructor parameters in Mailer.
• Remove FileMailer configuration and writeToFiles parameter from package configuration.
• Make psr/event-dispatcher dependency optional.
• Use file ID for attachments and embedded files.