If you’re on the latest version of the Mac OS, El Capitain, there’s an update to fix a bug with email.
“The current version of OS X El Capitan is 10.11.1, which was released on October 21, 2015. OS X 10.11.1 is the first update to the El Capitan operating system and it fixes several bugs that were discovered in the first version of the software.
OS X 10.11.1 improves compatibility with Microsoft Office 2016, fixing an issue that caused the software to crash, and it resolves two Mail bugs, one where outgoing server information could be missing and one that prevented the display of messages and mailboxes.
It also fixes an issue that prevented certain audio unit plug-ins from functioning properly, improves VoiceOver reliability, and adds more than 150 new emoji characters.”