SMTP client tests overhaul #76

Job	Run time
Test with Codecov coverage (ubuntu-latest / 1.23)	57s
Dependency review	11s
golangci-lint (1.23)	39s
Go vulnerabilities check	26s
REUSE Compliance Check	15s
Test with SonarQube review (ubuntu-latest / 1.23)	1m 39s
Test on FreeBSD 13.4'	2m 12s
Test on FreeBSD 14.0	2m 28s
Test on FreeBSD 14.1	2m 12s
Test (macos-latest / 1.19)	23s
Test (macos-latest / 1.20)	23s
Test (macos-latest / 1.21)	19s
Test (macos-latest / 1.23)	25s
Test (macos-latest / 1.22)	20s
Test (ubuntu-latest / 1.19)	23s
Test (ubuntu-latest / 1.20)	26s
Test (ubuntu-latest / 1.21)	22s
Test (ubuntu-latest / 1.22)	22s
Test (ubuntu-latest / 1.23)	23s
Test (windows-latest / 1.20)	21s
Test (windows-latest / 1.21)	22s
Test (windows-latest / 1.22)	22s
Test (windows-latest / 1.19)	17s
Test (windows-latest / 1.23)	19s
	16m 26s

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