Using MailViewed next to another email tracker.
Two trackers can't both control Gmail's Send button. MailViewed steps aside so your email always goes out; turn one tracker off to track with MailViewed.
Every Gmail tracker works the same way: when you press Send, it pauses Gmail for a moment, puts its tracking image into the email, then sends. Some trackers go further and take over the Send button completely, sending the message through Google's servers themselves. Two extensions can't both do that to the same click, and the loser is your email: it sits in the compose window and never leaves.
MailViewed refuses to let that happen. When it sees another email tracker active in the same Gmail, the compose pill turns amber and reads Paused · other tracker, and MailViewed leaves Send alone. The one exception is Pro's Individual copies for group emails: if it's on and you send a new email to two or more people, all in To, MailViewed stops that first press and asks before it goes out as one group email, where everyone sees each other's address. If you choose Send as one group email, press Send again. Your email sends normally through the other tracker, just without MailViewed tracking. If a send ever stays in the compose window anyway, MailViewed tells you after a few seconds, and your next press of Send goes straight through: the same email is never held twice.
Open chrome://extensions and switch the one you're not using off (you don't have to remove it). Reload Gmail.
Open a new compose. The pill should no longer say Paused · other tracker. It reads Tracking on (green), or Tracking off (grey) if you've turned off 'Track opens by default' in Settings or switched off 'Track new emails' in the extension popup; click the pill to track that email. If it still says Paused · other tracker, the other extension is still running somewhere on the page; reload once more.
It should appear in your MailViewed dashboard within a few seconds of sending.
Turning off tracking inside the other extension's own settings usually isn't enough: some trackers keep control of the Send button even with their tracking switched off. The extension itself has to be off.