Run your huge list through a proxy checker or SMTP validator before OpenBullet. Remove dead emails. OpenBullet works best with a 10 million combo file (approx 300-500MB) rather than a 2 billion combo file.
cut -d':' -f1 combo.txt | sort -u
Unlike a standard password wordlist (one word per line), OpenBullet expects structured data depending on the config. openbulletwordlist
Weakpass is a massive archive of wordlists and combinator attacks. It offers pre-made combo lists sorted by language and type. You can download a text file containing user:pass and feed it directly into OpenBullet. Run your huge list through a proxy checker