Just to add, Thunderbird the Mozilla mail client is available in a Mac version. Postbox is a commercial variant based on Thunderbird and seems to get good reviews.
Priced at $30-$50, so not particularly cheap.
As others have said there is no need to install anti-virus provided you are sensible, but if you really must have it,
the free Sophos version appears to be very useable and AFAICT doesn't produce any problems. I've had it running for some 3 months without any bother and, in that time it has found absolutely nothing.
Before anyone says, having AV installed is a condition of being able to access my work VPN.