Mulberry is an email client that has a large number of features and can work with sending and receiving via Gmail (as of 2017).
Email could break when Google completely sunsets SSLv3/TLSv1 support for email.
Receiving email works over SSLv3, sending over TLSv1.
You will need to enable Less Secure Access for your Google Account.
It was a commercial client, but is now free for classic use per their website
Serial Number: SFREE
Registration Code: 8B37 34EE 2324 08CE
(11.28 MiB / 11.82 MB)
Mulberry 3.1.6a / compressed w/
Stuffit

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2017-03-26 /
3ec4300ae0aba2c3d0803c5358cc68a8bb3e3b94 /

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(10.18 MiB / 10.67 MB)
/ Binary encoded, use
Stuffit Expander

24 /

2017-03-26 /
a752e5a34bc87ceb97c7fcfd2e05288cc320c312 /

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(13.1 MiB / 13.74 MB)
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DMG image

9 /

2017-03-26 /
10f5d2078be1d2ebcf1817fa584079bbbd02b4f0 /

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(13.36 MiB / 14.01 MB)
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DMG image

0 /

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c092d2b7f7506590ab0a2ada04efc22db76c8320 /

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(18.13 MiB / 19.01 MB)
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DMG image

1 /

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4f30743d6cdc266f6ae92bca41a3eb4433218737 /

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(18.28 MiB / 19.16 MB)
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IBM PowerPC
Compatibility notes 
Architecture: 68k PPC x86 (Intel:Mac)
- Notes:
- Mulberry v2.0.8 (FAT) runs native on either 68k Mac or PPC. Mac OS's 7.1 to 9.2.2
- Mulberry v3.1.6 and v3.1.6a are native (Carbon apps) that run on either Mac OS 9 or PPC Mac OS X.
Both require a free registration code to activate (or see pic #4, above).
- Mulberry v4.0.5 and v4.0.6 are native PPC Mac OS X. Freeware, requiring no registration to use (only versions before 4.0.5, did).
- Mulberry v4.0.7 and v4.0.8 are Universal, PPC Mac OS X and Intel Mac native, requiring no registration to use. Version 4.0.8 is now free Open Source Software (Apache License 2.0).
I've successfully used Mulberry v2.0.8 in Basilisk II (build 142) running Mac SSW 7.1.1. Not tested by myself in other emulators and Basilisk II compatibility is noted for this version of Mulberry and Basilisk II, only.
The development of Mulberry appears to have stalled, the final stable released version is still at v4.0.8 (2007).
Perhaps this page can help to draw some attention towards that project.
Emulating this? It should run fine under:
QEMU