SMTP client tests overhaul #78

Job	Run time
Test with Codecov coverage (ubuntu-latest / 1.23)	1m 0s
golangci-lint (1.23)	37s
Dependency review	12s
Go vulnerabilities check	22s
REUSE Compliance Check	15s
Test with SonarQube review (ubuntu-latest / 1.23)	1m 42s
Test on FreeBSD 14.0	2m 28s
Test on FreeBSD 13.4'	2m 6s
Test on FreeBSD 14.1	2m 31s
Test (macos-latest / 1.19)	27s
Test (macos-latest / 1.20)	28s
Test (macos-latest / 1.21)	21s
Test (macos-latest / 1.22)	21s
Test (macos-latest / 1.23)	24s
Test (ubuntu-latest / 1.20)	24s
Test (ubuntu-latest / 1.19)	32s
Test (ubuntu-latest / 1.21)	22s
Test (ubuntu-latest / 1.22)	23s
Test (ubuntu-latest / 1.23)	27s
Test (windows-latest / 1.19)	27s
Test (windows-latest / 1.20)	26s
Test (windows-latest / 1.21)	27s
Test (windows-latest / 1.22)	27s
Test (windows-latest / 1.23)	26s
	17m 35s

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