If you own the server and control whether or not a message-deletion is truly a message deletion.
If you have a mail service provider that worries about data-integrity, they're probably also going to be backing up your mail spool for some number of days. And, if anyone's coming at them with a subpoena-like instrument (like an NSL), you're not going to know if they've been required to not only turn over backups, but dupe-on-arrival any emails that transit their system that are to/from you.
Overall, if you aren't doing message body encryption, whether you leave your message on the server or not can make little practical difference.
Lastly, depending who you're looking to protect your mail contents from, the message-body can be significantly less important than its metadata.