Been using thunderbird for a good few years, and it manages 17 addresses across 5 different domains with no issues,
The other nice thing is being able to split mails out into local folders, using rules for each address, so I can find (for example) all the information relevant to flights with a specific airline, or all the messages that have come in from the bank, in their own folder,
Another nice trick with local folders is being able to automatically delete messages that have been read after a retention period that can be set by folder, which is perfect for managing things like flight promotions that are time limited. If I put a star on a message, even if it's been read, it won't get deleted.